-filter_complex "[0:v]trim=300:1800,setpts=PTS-STARTPTS[v0];[0:a]atrim=300:1800,asetpts=PTS-STARTPTS[a0];[0:v]trim=2400:3600,setpts=PTS-STARTPTS[v1];[0:a]atrim=2400:3600,asetpts=PTS-STARTPTS[a1];[v0][a0][v1][a1]concat=n=2:v=1:a=1[out]" -map "[out]"