ffmpeg -i rtsp://ip/videoSub -i rtsp://ip/videoSub -filter_complex "nullsrc=size=320x480 [base]; [0:v] setpts=PTS-STARTPTS, scale=320x240 [upperleft]; [1:v] setpts=PTS-STARTPTS, scale=320x240 [lowerleft]; [base][upperleft] overlay=shortest=1 [tmp1]; [tmp1][lowerleft] overlay=shortest=1:y=240" -c:v libx264 -f mpegts udp://239.101.101.1:1234