I'm sorry I don't know for sure, but I think you can just DL and use it, leaving your original install alone. I have two of the default viewers installed -- other than the fact that I've never bothered to configure them to run simultaneously (which is possible if you have the resources) they both work fine. I did the dual install when SL made the default viewer run WindLight, as I didn't trust it (I tested it when it was beta). Once I tweaked the settings though it ran fine.
I have several alts I use to test stuff out too...but I log them on my laptop next to my desktop. Sometimes I chat with myself because I'm really weird that way. Oops did I say that out loud?
