Good point.

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.