The second file can be an mp4 instead of an mp3:
ffmpeg -i input.mp4 -i input.mp3 -c copy -map 0:0 -map 1:0 output.mp4
To concat to mp4s:
ffmpeg -i input1.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate1.ts
ffmpeg -i input2.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate2.ts
ffmpeg -i "concat:intermediate1.ts|intermediate2.ts" -c copy -bsf:a aac_adtstoasc output.mp4