just want to play two files side by side
ffplay -i inputleft.avi -vf "movie=inputright.avi [in1]; [in]pad=iw*2:ih:iw:0[in0]; [in0][in1] overlay=0:0 [out]"

combine seperate videos into one, side by side
ffmpeg -i inputleft.avi -vf "movie=inputright.avi [in1]; [in]pad=iw*2:ih:iw:0[in0]; [in0][in1] overlay=0:0 [out]" sidebyside.avi