I actually do this pretty often in my own web series where I'm playing multiple characters. I find it better to be creative with framing and positioning rather than thinking about complicated technical solutions. If you watch any of the clips below in which I'm interacting with myself, you'll notice there's usually a vertical line in the background along which I can simply crop the two images together in post. I just shoot the scene a few times, playing the different roles, making sure I don't cross that vertical line, then in Final Cut I simply crop the left side of one clip and the right side of the other clip. People always seem to think I'm doing crazy work in After Effects, but I really just framed it properly and used a simply crop.
http://www.kesevergreen.com/video/kes_ever...akfast_high.mov
http://www.kesevergreen.com/video/kes_ever...yspace_high.mov
http://www.kesevergreen.com/video/kes_ever...ressed_high.mov