ffmpeg dual webcam stream commandline and output

daniel@production ~ $ ffmpeg -f v4l2 -framerate 30 -video_size 640x480 -thread_queue_size 512 -i /dev/video1 -i /dev/video2 -filter_complex "
nullsrc=size=640x480 [background];
[0:v] setpts=PTS-STARTPTS, scale=640x480 [left];
[1:v] setpts=PTS-STARTPTS, scale=640x480 [right];
[background][left]       overlay=shortest=1       [background+left];
[background+left][right] overlay=shortest=1:x=320
" -pix_fmt yuv420p -f v4l2 /dev/video3
ffmpeg version 3.0.1 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
  configuration:
  libavutil      55. 17.103 / 55. 17.103
  libavcodec     57. 24.102 / 57. 24.102
  libavformat    57. 25.100 / 57. 25.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 31.100 /  6. 31.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
Input #0, video4linux2,v4l2, from '/dev/video1':
  Duration: N/A, start: 17223.188547, bitrate: 147456 kb/s
    Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 147456 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
Input #1, video4linux2,v4l2, from '/dev/video2':
  Duration: N/A, start: 17224.453327, bitrate: 294912 kb/s
    Stream #1:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 294912 kb/s, 60 fps, 60 tbr, 1000k tbn, 1000k tbc
Output #0, v4l2, to '/dev/video3':
  Metadata:
    encoder         : Lavf57.25.100
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc (default)
    Metadata:
      encoder         : Lavc57.24.102 rawvideo
Stream mapping:
  Stream #0:0 (rawvideo) -> setpts
  Stream #1:0 (rawvideo) -> setpts
  overlay -> Stream #0:0 (rawvideo)
Press [q] to stop, [?] for help
frame=  107 fps= 25 q=-0.0 Lsize=N/A time=00:00:04.28 bitrate=N/A speed=0.995x
video:48150kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
daniel@production ~ $