Open a video editor that supports layers or picture-in-picture
Import both videos into the project
Place one video on the main timeline
Add the second video on an upper track or overlay track
Resize and position each video within the frame
Adjust the crop if needed to fit both videos side by side or top and bottom
Sync the clips if you want them to play at the same time
Add borders or spacing between the videos if desired
Preview the combined frame
Export the final video in your preferred format