Well I hope someone has a better answer but here is the easy way.
Setup your camera on a tripod or something completely still. Film yourself in camera left, talking to an invisible you camera right.
Now switch places, film yourself in camera right, interacting with invisible self in camera left.
Now in FCP, drop the two clips on top of each other. One should have you frame left, the other frame right. Now crop the top clip about half way.
Now you should see two of you on the screen, timing & acting will make the rest work. If you get any lines in the middle from the top clip, try to feather the edge a bit.
BAM, your seeing doubles.
Will Keir