To combine two videos with frame size for example 400×300 you can use (again) the ffmpeg command:
ffmpeg -i <moviename1.avi> -vf "movie=<moviename2.avi> [in1]; [in]pad=800:300:color=white[in0]; [in0][in1] overlay:400:0 [out]" -crf 5 -b:v 8M <moviename3.avi>