I have been known to do this as well, but I have also found that I am more creative when I have more than one thing going on. Generally I find that a solution in one application will lead me to rethink a problem in the other.

That being said I also go through times when I want to work on both or use one to procrastinate from working on another one.

So you are not alone. I have found the best system that works for me is setting small goals and deadlines. That gives me the flexibility to move from project to project while also feeling like I am accomplishing something.