Thomas,
Here's a reply I made in another thread April 25. You would have found it had you searched for it:

<<<Here's a method that works with Vegas 4, but because it uses only track motion, I suppose it will work in Vegas 5 as well.

To get split screen with unsqueezed images in Vegas:
Track one, open track motion. Set Center values to 0 and 240 (288 for PAL). This shifts the frame left.
Trach two, open track motion, set Center values to 720 and 240 (288 for PAL). This shifts the frame right.
Use Pan/crop to shift the images so the action is within the narrowed frames (You only get half pictures).

If you wanted one on top of the other, the settings would be (in PAL) 360 x 0 for the upper and 360 x 576 for the lower. I'm still talking about unsqueezed frames, so half of each will be left outside of the visible area. >>>

I might add: If you don't want half and half frames but more like 1/3 - 2/3, do your own math. Also: If you do want squeezed images, work on the size values in track motion, instead of the center values.
Tor