When I briefly tried making more than one cartoon at once to get my ass to do something I found that it worked pretty damn good. It helped during those times when you just can't focus on making your movie because your mind is thinking of another movie or something else. When you reach that point where you want to take a break and work on something else, or play a video game, you have another movie to work on.
I'd advise keeping them in different production stages though. For example, and this applies to your shit very well, if you have one movie where you're working on an intense FBF heavy fight scene and one day you just don't feel like animating three million frames, you can work on another movie that needs some backgrounds done or some ambient shots finished. Or maybe you can draw out the animatic of something. The same works in reverse too, for those times when you NEED to animate some people getting their asses beat instead of lip syncing a plot moving scene.
Just don't do more than two at once, because then you'll be like me and have dozens of started projects at various stages of completion and very few finished products.
Gianni
Don't make promises you can't keep!
JazLyte
So true.