Why do Vishnu and Shiva have several common names?
From: paruchuri sivaramakrishna
even vishnu have name “parameswara” any ways siva had unique name “maheswara” ..but there are nearly 15 to 20 names common to vishnu and shiva..like  krishna govinda gopala gopati siva  rudra sambhu hari hara brahma sthanu  sahasraksha sahasrapath kamadeva  mahadeva  devadeva vishnu  prajapati and list goes on  why is this?
Answer: When we understand the concept of the absolute truth and the manifestation of that absolute truth at various levels, it is always important to keep in mind the principle of achintya beda abedh. Achintya beda abedh is simultaneous inconceivable oneness and difference. So as far as the common names are concerned, so that verse states etena sarve vyakatha vyakathaha, that etena sarve it is this absolute truth who is at rest by vyakatha vyakathaha, it is he who is addressed by all names. So there is one absolute truth who is referred to by many names. So if we look at the various names, are both proper nouns describing certain persons as well as common nouns or they are describing certain qualities of those persons. So for example shiva as I said can refer to auspicious and it can refer to the person name who has the name shiva.
So now certainly there is a person name shiva, at the same time, that the supreme lord who is all auspicious and the auspicious aspect of that supreme lord is manifested through a particular personality name shiva. So in that sense because lord shiva manifests many of the extraordinary qualities of lord Vishnu, so he also has names which reflect the qualities of the supreme lord. So that means auspiciousness ultimately is a quality of lord Vishnu, he is supremely auspicious Om apavitra-pavitro-va sarva-vastam gato api va yah smaret pundarikaksham sah bhayabhyantara suchih Sri Vishnu Vishnu Vishnu. So it is described in famous mantra that by the remembrance of lord Vishnu, his pundarikaksha, his lotus eye, one can become purified internally and externally, he is the supreme purifier, so in that sense he is auspicious.
At the same time this supreme lord manifests himself as lord shiva by contacting himself slightly with the mode of ignorance and thereby he specially purifies those who are in the mode of ignorance and those who would not be inclined to worship lord Vishnu. So in that sense for those are living inauspicious life lord shiva is a manifestation of lord vishnu’s mercy by which auspiciousness can be brought into the lives of those who are living inauspiciously. So in that sense the auspicious quality of lord Vishnu is manifested through lord shiva and that’s why that name is valid for him. Similarly there are many other qualities of lord Vishnu that are manifested by lord shiva. Srila Rupa Goswami in the Bhakti Rasamrita Sindhu based on an exhaustive analysis of the Vedic literature explains that various qualities of the supreme lord can be innumerated as 64 qualities. So now among these Krishna is the manifestation who manifests 64 qualities, other Vishnu manifestations manifests 60 qualities. Lord shiva manifests 55 qualities, lord brahma manifests upto 50 qualities. So lord shiva manifests 55 of the qualities that lord Vishnu manifests and since many of the names are related with the qualities that are manifested by lord shiva, but which are qualities, they are also possessed by lord Vishnu, so naturally the names will overlap.
So for example lord shiva is famously known as vishvanath, so he is the lord of the universe because durga devi, his consort she is incharge of the material world and he is her lord. So in that sense he is the lord of the material world. So there are ofcourse certain names which are reserved for lord Vishnu for example anantha koti brahmanda nayak, lord shiva is generally nor referred to by that name. So parameshwar, so if you see in that way it refers to the great eshwar, it refers to the transcendental eshwar, it can refer to the supreme eshwar. So now lord shiva is supreme within the material world, there is no one greater than him and lord Vishnu is transcendental to the material world. So in that sense if we look at the overall philosophical understanding then we can contextualize and understand these names as referring to specific attributes of lord Vishnu which are also manifested by lord shiva.
Srila Madhvacharya in his brahmasutra bhasya explains that actually lord Vishnu is sarva namavan he is sarva nama van, van means possessor like we have danavan, possessor of  wealth so lord Vishnu sarva namavan, he is the possessor of all names, why because ultimately all qualities manifests from him. So certain qualities may be also manifested specially by certain other personalities, exalted personalities like the devatas and certainly one of the most exalted of the devatas, that is lord shiva. So that is how they many have some common names. In general now if we look at lord shiva being called as maheshwara, well fine maheshwara means that great eshwara, he is certainly the great eshwara, there is no doubt about that. At the same time, that doesn’t mean that there is a competition between shiva and vishnu, who is supreme, no, there is a natural harmonious relationship between the two of them and in general this relationship is something which our simplistic categories find difficult to understand. See for example we had the categories of, when the religions when they came to India, they found that they had the simplistic categories in their mind from their own historical tradition.
When britishers came to India and Europeans came to India they had the experience of Christianity as the living religion of their time which was monotheistic and Christianity was presided by the roman empire and the roman culture was polytheistic, so the had these only two categories, they had monotheism and polytheism and in their idea in polytheism actually the many gods were always competing for supremacy, so when they came to India and they saw actually India has many gods, so they said you are not monotheistic that means you’re religion is polytheistic. However if we look at the Vedic tradition, its not the divine world, means the world which includes lord as well as the other devatas, it is not a world where everybody is _______ for supremacy, they are not competing it is a cooperative network like the administration of the government through the cabinet ministry headed by the prime minister.
So actually speaking, the Vedic system is not just monotheism or polytheism, it is actually multi level monotheism, multi level monotheism means that there is only one god but he can be worshipped at different levels and different levels means he has many manifestations who are not exactly him, there are his representative but by worshipping them we are indirectly worshipping the supreme lord and among these multilevel manifestations lord shiva is at the highest level, that means he is the most intimately close to the supreme lord who has relationship with him, so that’s why it’s not so simple that we use our categories from ordinary experience and we try to super impose the word Vedic hierarchy. So first of all it is not only multilevel monotheism it is also polymorphic monotheism, what does polymorphic means morpho means form, poly means many, so polymorphic means that there are many forms of that god, Advaitam achyutam anadir anantarupam, so the one lord Krishna manifests in many many many forms and one of the forms of lord Krishna who is there in the spiritual world as described in Chaitanya charitamrt is actually the aradhya dev of lord sadashiva that is sankarshan.
So actually sadashiva is lord shiva in his eternal manifestation who exists in the junction of the material and the spiritual world at the boundary is one forth in the spiritual world, three forth in the material world and at that manifestation he is eternal so there is expansions of lord shiva who are in the material world who are temporary, rudra, there are eleven rudras who are temporary, when the world gets destroyed they become unmanifest but there is sadashiv who exists beyond the material world, he is eternal, he is very very close to the supreme lord and because of this proximity he is the most intimate or highest manifestation of non difference of achintya bedha abedh with the supreme lord. So lord shiva is not lord vishnu, at the same time he is not entirely different from lord Vishnu, he is a manifestation of lord Vishnu, who is connected with the mode of ignorance as we have pointed out earlier.
So in this way if we understand the broad philosophy and within it we understand the points that all qualities emanate form the lord and therefore all names which are based on various qualities ultimately belong to the lord, so sarva namavan as Madhavacharya says etena sarve vyakatha vyakathaha, as Vedanta sutra says and then certain personalities manifests specific qualities and therefore they many also have the same names as the supreme lord. So lord shiva is the highest among all the devatas and as he is very close to the supreme lord, so he has many of the names, many more names than most of the other gods also which are he same as ________ as the supreme lord and we also discussed how simple categories like polytheism and monotheism don’t really describe the profound Vedic conception of the absolute truth and the various manifestations that are there of that absolute truth and these manifestations, the various levels of difference and non difference that they have with there absolute truth. So if we still have doubts or questions if we keep practicing Krishna consciousness and praying to the supreme lord and if we have had some past attraction or cultural devotion to lord shiva we can also pray to lord shiva to seek his guidance and get you a clear understanding of these things and by their combined grace, we will be able to get a clear understanding of it all.
Thank You.