FFMPEG : Merging Two Audio Files
Since you wish to alter effective length of the tag, skip the amovie looping and use the aloop filter after padding the audio to 10s. 

ffmpeg -i main.mp3 -filter_complex "amovie=tag.wav,asetpts=N/SR/TB,apad=whole_dur=10,aloop=-1:1M:0[tag]; [0][tag]amix=duration=shortest,volume=2" -vn tag-out.mp3