well, yes. Amount will exactly double - so value should halve at least. (formally one coin with one cap - then two coins with less cap combined as everyone with a brain will cash out before the madness occures)

A pair Gavincoin/Mpcoin for the direct warfare can be expected on some exchange. It'll be fun to watch but certainly nothing anyone wants to get caught up in. War could go on for months but eventually both coins become worthless in the end and will be replaced with altcoins.

Maybe people get good arbitrage opportuninty in case chaos isn't too omnipresent to even move coins on any of the two chains safely.

Probably 51% attacks from either side can be expected aswell as wild fluctuations not only in price but also in hash of both coin. It will be an epic act of cannibalism. So, really noobs should stay away from the mad people. This isn't even fun. Better buy yourself Litecoin and dogecoin already and dump btc on the next peak like everyone else.