I could be wrong, but I'm pretty sure OP meant an entirely new cryptocurrency (let's call it, 5coin) with a new block chain and everything, but one 5coin would always be equal to 5 bitcoins. If that is the case, it would very hard (pretty much impossible) to keep the two currencies in sync. Maybe if there was someone that had a shit ton of both currencies, and always traded 5 Bitcoin/5coin, but the chances of that are pretty slim too.
Ask Norman Lamont how trying to force a fixed exchange rate goes. I think I just heard George Soros chuckle.