ffmpeg passthru, bash script: copy audio and video from different sources to a new container

Something went wrong when encoding a video archive. All videos exists as FLV and MP4 files (and has got the same name before the extension). The MP4 files are higher in resolution but contain no audio, but the FLV files has great audio.

So I would like to use ffmpeg's passthru function and copy the audio and video streams to a new MP4 container, but pick the streams from two different files. Audoio from FLV and Video from MP4.

audio FLV + video MP4 > new MP4