Maybe this will help someone else. I came across an easy way to create a split screen special effect using the Composite transition. This also can be used to make custom image masks.

For a split screen effect:
1. Place your first video on track 1. Place your second video on track 2 underneath it.
2. Add a transition and stretch to the full duration of your videos.
3. Click on the transition, change it to Composite. Scroll down beneath the red box...beneath Alpha Channel Operations...go down to where it says Wipe File.
Choose the wipe "linear_x.pgm".
4. Go back up. The Opacity setting no longer controls opacity, but rather the percentage of the Wipe. For instance, an Opacity setting of 50 will actually freeze the wipe at 50 percent, showing half of track 1 on one side and half of track 2 on the other.....a setting of 70 will freeze the wipe at 70 percent.
5. Adjust the softness of the wipe to make the transition smoother.

You can also create custom image masks. Simply place your mask in the Kdenlive wipe folder and it'll show up in the list of wipes (you may need to restart Kdenlive for it to show up). You can then choose it and use as a mask.

very cool! thanks for sharing.