This should give you an idea...
Place a blank clip on track 1 (this will be the background)
Overlay mobile video 1 on track 2, directly above
Overlay mobile video 2 on track 3, directly above
Drag the right handle of the blank clip to the same length as the mobile video clips
Click the FX button on clip 1 and apply the SPLIT SCREEN effect, choosing HORIZONTAL 2, REGION A, FIT TO REGION.
Do the same with clip 2 but click REGION B.
The result 