ffmpeg -i background.mp4
       -i overlay.mp4
       -filter_complex "[1]setpts=PTS-STARTPTS+162/TB, scale=512:288 [over];
                        [0][over] overlay=748:412 [outv];
                        [1:a]adelay=162000|162000 [audio];
                        [0:a][audio] amix [outa]"
       -map "[outv]"
       -map "[outa]"
       -codec:v libx264
       -crf 21
       -bf 2
       -flags +cgop
       -pix_fmt yuv420p
       output.mp4

ffmpeg -i background.mp4
       -i overlay.mp4
       -filter_complex "[1]fifo, setpts=PTS-STARTPTS+162/TB, scale=512:288 [over];
                        [0][over] overlay=748:412 [outv];
                        [1:a]adelay=162000|162000 [audio];
                        [0:a][audio] amix [outa]"
       -map "[outv]"
       -map "[outa]"
       -codec:v libx264
       -crf 21
       -bf 2
       -flags +cgop
       -pix_fmt yuv420p
       output.mp4