Right. But economically speaking, if exchanges started allowing you to trade both coins against each other, then supply has doubled while total Bitcoin demand remained constant. So the price would tank to at least 50% of its previous level on each side of the fork to reflect the new equilibrium.

Anyway, I think this is extremely unlikely to happen, because:

1) It requires 50% of miners to cooperate to even create a >1MB chain

2) Anyone using the old chain would be insecure, due to lack of mining power making it susceptible to a 51% attack.

3) If that many miners were on board, it's unlikely that existing core developers would opt to continue working on an insure fork rather than admit defeat.

4) If that many miners were on board, it's unlikely that exchanges would opt to confuse users by letting them separately trade two different types of Bitcoin, rather than just switching to the more secure, more popular chain.