I just dont understand this at all.
You want 2 videos of the same scene.
You need 2 ways to record this.
For some reason you are only interested in using a webcam to capture? (yes, the webcam captures, the mac records....in the same way a camcorder captures and the tape records)
If you use 2 webcams, youd need 2 macs. With 2 camcorders, and in essence, 2 tapes, you capture the footage on 1 tape to the mac, then the 2nd tape. Now you have 2 video files you can import into iMovie, FCE, FCP, AE, what have you.
Like all non-linear video editing software, its easy to place one video on one track, another video on another track. Then shrink/move/whatever the videos to give you split screen. Thats the idea behind non linear editing, playing multiple tracks at one time non-destructively.
Whats so hard to understand?
I think you think the only way to get video onto a computer is to record live?