Is mining faster with 2 GPUs in crossfire or with 2 PCs with one GPU each? Is it faster to have 2 separate computers with the same graphics card but different specs, or one computer with 2 GPUs in crossfire?

By math, they should perform the same im guessing at the same clock speeds
They do indeed perform exactly the same

Crossfire is not encouraged as it gives no advantage and may impact your overall performance. The computation is done in parallel on each card so there is no need for them to communicate among each other.