Not sure why / how.
a) Either of these attributes would have to be supernatural / imaginary. I don't know everything about anything. Thus the notion of knowing everything about everything is a conspicuous impossibility *, with one exception.
b) Either of these attributes is impossible, thus supernatural, conceptual rather than practical.
So since each is impossible is combining the two merely twice impossible? 2 x ∞ = ∞
Therefore not clear why combining these two impossibilities renders them more impossible.
The exception mentioned above:
The omniverse may not be a duplicate of itself, but it is itself. Thus the omniverse knows itself in the sense of manifesting itself. Therefore some mysteries about the omniverse may be unknown, but not necessarily unknowable. The omniverse knows. In that sense it knows all. Thus, is the pantheist god not omniscient?
* And in an infinite universe, omniscience means infinite knowledge.