But let's say we have Chain A with 51% and Chain B with 49%. If the miners on Chain A want to attack Chain B, they'd have to move their computing power onto Chain B. In which case the miners from Chain B, could attack Chain A while they're busy doing their attack on Chain B.

In the end, it would be MAD (mutually assured destruction) unless the miners from one chain had enough to defend theirs while attacking the other. For that you'd need at least 66.6(repeating, of course)% miners on Chain A with half of those able and willing to perform an attack on Chain B.

I think this would be hard to achieve.

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.

I read your blog post, but I didn't see where you mentioned that, so it's a little hard to argue with. I do disagree, however.