If chains forked, can the same asics mine on either chain?
That depends on why the chain forked.
If so, could a majority miner attack the weaker chain?
Sure. But to do so, they'd have to abandon mining on the chain that they aren't attacking. Since they were a "majority miner", that would leave their preferred chain much weaker and open to attack itself.
Could the weaker chain modify their own protocol to brick the asics?
![Kiss](https://bitcointalk.org/Smileys/default/kiss.gif)
Brick? You mean damage them? Probably not. But they could adopt a protocol that the current bitcoin ASICs would be useless for. Those ASICs could still be used on any other coins or forks that used bitcoin compatible mining.