Question Split Screen Scenario?

I enjoyed working with the split screen on a couple of projects, and it worked fine on two short (matched in length) videos. I, however had less success trying to add a short split screen in the middle of a larger video file. I was able to get the short video to split, but the longer (primary) file was split through it's entirety after adjusting the track motion. Any ideas how to make an insert without disrupting the whole video?

Thanks,
LarryC