Who is Ishwar according to Advait Vedanta - Lord Shiva or Vishnu?
Yeah, there can be only one Saguna Brahman as the following verse maintains-
B.Su. 4,4.17. (The released soul attains all lordly powers) except the power of creation etc., on account of (Iswara being) the subject-matter (of all texts where creation etc. are described), and (the released souls) not being mentioned (in that connection)...Moreover, this would lead to many Iswaras, which may give rise to a conflict of wills with respect to creation etc. Therefore the powers of the liberated souls are not absolute but limited, and are dependent on the will of Iswara.
But it seems Saguna Brahman of Shankara Vedanta is distributed in nature rather than a centralized one.
B.Su 4.3.8...“And conducts them to the worlds of Brahman” (Brih. 6. 2. 15). The plural number is not possible with respect to the Supreme Brahman, while it is possible in the case of the Saguna Brahman, which may abide in different conditions.
So when Saguna Brahman is distributed in nature, it can simultaneously be present as various forms like of Vishnu, Shiva, Brahma, etc. Therefore, all can be different manifestations of the same Saguna Brahman parallely. Let me end the answer with a verse of Maya Panchakam by Adi Shankara.
बत विरचय्य बुधानपि प्रकामम् ।
नघटितघटनापटीयसी माया ॥ ५॥
5. Alas! Maya, which is adept at making the impossible happen, creates in Brahman which is homogeneous, without any parts, distinctions such as Brahma, Vishnu and Siva and thereby perplexes even the learned by making them look upon Brahma, Vishnu, and Siva as different from one another.
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