Using PE9 I cant see how to have one track on the left hand of the screen and the other on the right hand in a fixed split screen like a horizontal wipe frozen half way through.
I wanted to use it to exactly synchronise two tracks of the same material shot with 2 different cameras from different angles and show the interviewer on one and the interviewee on the other.
If you video is on the Video 1 track and the other is on the Video 2 track, directly above it, you can right-click on each clip and select Show Properties.
In each clip's Properties panel, open the Motion clips and change the Scale. (50% is a good place to start.) You can then drag each clip into the area of the video frame you would like it to appear and further tweak the Scale properties.
As an alternative, you can just apply the Crop effect to the clip on the Video 2 track, right-click on it and Show Properties and, in the Properties panel, set one side's cropping to 50%.
Or you can combine Crop, Scale and Position.
There's lots of how-to's, step by step, on creating these effects in my "Cool Tricks & Hot Tips for Adobe Premiere Elements."