Yeah you're right, it gets weird with close calls.

pro A: 51% pro B: 49%

If B miners move to attack A, it means B will be successfully double-spent sooner. I'm not sure how best to coordinate the attack vs defense, but pretty sure ultimately the majority would win out. And yes, there is mutual destruction in play.

This is less of an issue with say, 67% and 33%.

Anyways, I also argue that there's only room for 1 secure PoW blockchain, and that's basically Bitcoin.