Because they are separate. They can be unique and not contradict each other.
If they both exist outside of time and space and are both all-knowing, then they must know of each other, correct? And they must each know that they are equally perfect. So why do they choose to create separate universes? If they do not disagree with each other in any way, shape, or form (which they logically cannot, what with both of them being absolutely perfect), why not just create one hypergigantic superuniverse and rule it together in all their perfection?
In a nutshell, they would obviously have had the chance to work together, and instead they chose not to. To me, this implies some difference of opinion. Does it not?