Infinite vs Infinite?Well what happenswhen an unstoppable force meets an immovable object? Nothing... They cancel eachother out.
U-DO is only infinite in the context of its own universe. The Void is beyond the universal scale, as is time compression Ultimecia. Two multi universe destroyers cancel out an entity bound to one universe, as he would be destroyed along with the rest of the universe if either of these two attacked it.
Again, character claim. U-DO has infinite power too according to characters.
And its only "infinite" in its own universe. If something more powerful than one universe wiped out its plane of existence, it too would be destoryed.