Arbitrage in the Bitcoin / Bitcoin Cash fork So assuming BTC and BCC are both viable and tradeable for some period after August 1st, is it not inevitable that some people will double spend, by trading a coin on BTC and also the same coin on BCC?

Any ideas what that will look like from an arbitrage perspective while the fork works itself out?
A coin on either fork is distinctly a coin on its respective fork. Sure you can send someone BTC and then send someone else BCC that spent the "same" input, but no one will care because that is entirely a different coin using a different blockchain. With the kind of fork that will be happening, no sort of fraud (outside of any normal fraud) can occur. The fork implements two way replay protection so receivers cannot replay transactions and get money on both forks, and double spending cannot happen because this will be a persistent chain split with no possibility of blockchain reorganizations.