Hello there!
I've worked on numerous project at once. I'm dong it right now actualy.
I'm workin' as a pixel artist for a console game project, I'm taking care of coding/design/art for a personal game, I'm drawing a monthly comic and working on Warcraft 3 custom maps. If you would've asked the question last month I would've told you that I was also finishing another console project, working as artist and designer but the project was recenlty finished.
It's not very difficult to do all things at once but you have to know your priorities. You can't do everything at once, even if you want to. You have to make sure you know what's more important to your eyes and schedule your time accordingly.
Like mentionned in earlier posts, anything you're doing with someone else should be in your top priority because, and I'm talking out of experience here, when a member of a team doesn't care much about what he/she's doing, it demotivates the others and may lead to a crash of the project. Even if all members of the team are highly motivated, it's going to make the project fail or will result in the abolition of the unmotivated member.
Another thing: personal projects tend to be easily thrown in a drawer. You probably work/go to school (which is as time consuming) and you probably have a family and friends. You probably have girlfriend/boyfriend you wanna hang out with sometimes too. Working on side projects means you have to schedule when your working and when you're socializing. The more projects you have, the more you're dividing the time you can spend on one project, leading to less palpable progress. And of course, less palpable progress leads to demotivation, leading to the ditching of one or all projects. And sometimes, it will affect you psychologivaly, making you think you can't achieve anything or that you're not goog enough (I'm not kidding: I've seen it happen).
It's not about how good you are or how you are talented. It's all about maturity and discipline. You see, I mentionned everything I do at the same time at the beginning of my post but I don't do everything all at once. every week, I spend a couple hours working for the pixel art project. Every 2 weeks, instead of working on the project I work on my personal game. I take one week of the month off from all projects to write and illustrate my monthly comic. Once every 2 months or when I have some sudden inspiration, I work on my warcraft 3 map. And, whenever I get ideas for any projects, I write them down in a note pad. This way, when I start working back on a project, I have fresh stuff to work on and I'm not spending time thinking baout it but actually doing some stuff on it.
I make sure I spend time with my girlfriend too. She supports me in what I do. If I did not have her, I would simply go insane (some say less insane but that's another story
). I make sure I spend all the time possible with her and my firends. I also try to entertain myslef accordingly. Too much work doesn't help imagination which is the key to game and graphic design.
So anyway, good luck with your projects!
Have a good time doing them and remember, fun before labor and one finished project is worth a dozen half-finished.
Take care!
Guert