Do you ever work on a few themes and switch between them on your local installation of wordpress? Should this be ok switching between the themes when working on them?
I don't see any problem with it. It's just like reading two books at once. You can do it if you want, nothing bad is going to happen. I personally like to finish what I started before moving on to any new ventures but if you want to work on 2 for whatever reason, go for it.
Thanks zeldaze, actually you made a good point, its probably a better idea to finish one before starting the next.
Yea, that way once you finish your first one you can take a look at it and see what aspects you liked and what ones you didn't then when you make a new one try to use the same styles of the good components that you found and try to improve on the things which didn't quite work out.