Ok, first you need the background image and both video clips imported into a project. Sorry, I tried to do a video screen capture, but this computer is getting pretty slow and just wouldn't do it.
Place the background image in track 1.
Then, place one of the video clips in track 2 and adjust to ahlf screen.
Last, place the second video clip in track 3 and adjust to the other half of the screen.
To adjust, highlight the clip in the time line and there will be corner markers in the preview screen. Just grab them and drag. You can also make more detailed placement in the PiP Designer (modify button). If the clips are not the right shape, you can also apply a mask, also in the PiP Designer.
If you need further help, just ask.
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CORNBLOSSOM