Both the visvarupra of Shiva and Krishna are identical, the omniform can be better understood by understanding the following verses of Vedas first.
Sarvam Khalvidam Brahman' 'All this is indeed Brahman'
- Chandogya Upanishad 3.14
In the beginning this was Self alone, in the shape of a person
(puruṣa). He looking around saw nothing but his Self (Atman). He first
said, "This is I", therefore he became I by name. —Brihadaranyaka
All the avatars are of same Purusha/Brahman/Paramatma/Supersoul who is omnipresent as souls of all beings. People are just using their bodies and earth for a period but are ignorant of God around them because of their temporary egos of temporary body, born with the power of avidya Maya, they have not created their bodies nor the world around them. Who is the creator and cause of everything? That alone is depicted in Visvarupas shown by various avatars like Shiva, Krishna etc., taken by one formless Brahman since worship of formless is difficult than worship of God with a form. Infact the ever changing inert material universe is created alone to understand the formless eternal Supersoul Purusha. Sanatana Dharma is a Henotheist religion where the hymns of the Rigveda, the oldest scripture of Hinduism, mention many deities, but praises them successively as the "one ultimate, supreme God", alternatively as "one supreme Goddess", thereby asserting that the essence of the deities was unitary (ekam), and the deities were nothing but pluralistic manifestations of the same concept of the divine (God).
They call him Indra, Mitra, Varuna, Agni, and he is heavenly-winged
Garutman. To what is One, sages give many a title.
— Rigveda 1.164.46
BG 5.18: The truly learned, with the eyes of divine knowledge, see
with equal vision a Brahmin, a cow, an elephant, a dog, and a
Brahman/Atman was also explained by Yagyavalkya, one of the wisest sage in the courtyard of Janaka after defeating other learned scholars of Vedas. Brihadaranyaka Upanishads
3.8.9. Under the mighty rule of this Immutable, O Gārgī, the sun and moon are held in their positions; under the mighty rule of this Immutable,
O Gārgī, heaven and earth maintain their positions; under the mighty
rule of this Immutable, O Gārgī, moments, Muhūrtas, days and
nights, fortnights, months, seasons and years are held in their
respective places; under the mighty rule of this Immutable, O Gārgī,
some rivers flow eastward from the White Mountains, others flowing
westward continue in that direction, and still others keep to their
respective courses; under the mighty rule of this Immutable, O Gārgī,
men praise those that give, the gods depend on the sacrificer, and the
Manes on independent offerings (Darvīhoma)
3.8.10. He, O Gārgī, who in this world, without knowing this Immutable, offers oblations in the fire, performs sacrifices and undergoes
austerities even for many thousand years, finds all such acts but
perishable; he, O Gārgī, who departs from this world without knowing
this Immutable, is miserable. But he, O Gārgī, who departs from this
world after knowing this Immutable, is a knower of Brahman.
3.8.11. This Immutable, O Gārgī, is never seen but is the Witness; It is never heard, but is the Hearer; It is never thought, but is the
Thinker; It is never known, but is the Knower. There is no other
witness but This, no other hearer but This, no other thinker but This,
no other knower but This. By this Immutable, O Gārgī, is the
(unmanifested) ether pervaded.