Śrīla Bhakti Rakṣak Śrīdhar Dev-Goswāmī Mahārāj explains the identity of Śrī Advaita Āchārya.
Student: How is it that Advaita Āchārya can be Mahāviṣṇu and Sadāśiva?
Śrīla Śrīdhar Mahārāj: Sadāśiva and Mahāviṣṇu have similar positions. The jīva-tattva as a whole is represented in Śiva. If the whole jīva-tattva is represented in a symbolic way, that is Śiva, and Śiva has a twofold characteristic, adaptability for two sides. One, he can exploit this misconceived world, and two, when he goes to serve the real world, he is called Sadāśiva. That is the more original representation of the form of Śiva, there in Śivaloka. Here, Śiva is utilised as the first officer to serve in this mundane world, the misunderstood world. That is, Nārāyaṇ enters here in that aspect, that function, and manipulates things through Śiva. So, if we are to look at the manipulator, from where the inspiration is coming, then we see Mahāviṣṇu, and if we are to look at through whom it is working, then we find Śiva. The delegation of Mahāviṣṇu comes in Śiva as an eternal post created at the bottom of this whole brahmāṇḍa, of the misunderstood aspect of the world. Śiva has the eternal position of an officer where the delegation of Viṣṇu in there in the creation for the management of this world. When we look at the inner function, Mahāviṣṇu is there utilising Śiva, the highest representation of this māyic world, the jīvas as a whole.
Whenever any South Indian people approached me while I was there in Madras and asked, “Is Śiva greater or is Viṣṇu greater?” Then I used to point out that Śiva is a searching type: he is practising penances and searching after truth. That means he is not complete. But Viṣṇu is always full. Generally, a Viṣṇu-avatār is seen to be full within Himself and not searching for any higher promotion or higher object. Śiva is the type who is searching, enquiring, and making penances to attain his end. So, he is in want, and he is trying his utmost to eliminate his inner want, but Nārāyaṇ does not do this. He is satisfied. He is full in Himself, and we see generally also that when there is a problem that no one can solve, then at last it comes to Nārāyaṇ, and He comes and solves the matter. His position is like that. He is self-content, and He is the giver of the highest solutions in very intricate cases which cannot be solved by anyone else.
Source
Spoken 19 February 1982.