Let me be clear I think it's highly probable that the price post fork will decrease. I am less convinced that it will plummet like BCH did. This is an uncontentous planned network upupgrade that everybody knows needs to happen.
Anyone thinking that they are buying into two coins is wrong. Post fork the old currency is going to 0. No one will want to mine it. Mostly because of the difficulty bomb that will explode.
Well, that's the plan.