Picture in picture
Picture-in-picture is one clip on top of another, which needs layers on a timeline. The full editor does it — or use the collage tool here for a simple side-by-side.
PiP means two clips at once, one inside the other. Layers are what a timeline provides.
In the editor the inset can be positioned, scaled, rounded and keyframed to move.
If you only need them next to each other rather than one inside the other, the "video collage" tool here does that in one pass.
FAQ
Side by side instead?
The collage tool on this site does that directly.
Can the inset move?
Yes, with keyframes.
Which audio plays?
Both, mixed, with independent volume in the editor.
Can it be rounded or have a border?
Yes, both.
Is it free?
Yes.