Check out the tutorials listed here. https://www.google.com/search?client=firefox-b-1-d&q=Videopad+split+screen
In short...
Add a clip, choose the split-screen effect.
In the drop-down box, choose the desired split (2x2, 4x4, 5x5 grid, etc.).
Assign the clip to a box (A, B, C, etc.)
Choose whether it is to stretch, fill, or fit.
Overlay a second clip and follow the same steps.
Assign it to a different letter (box).