>ffmpeg -y -f video4linux2 -vcodec mjpeg -s 1280x720 -r 30 -i /dev/video1 -f video4linux2 -vcodec mjpeg -s 1280x720 -r 30 -i /dev/video2 \
-vf '[0:v] pad=iw*2:ih [p]; [1:v][p]overlay=w[out]' \
-map '[out]' -an left.avi

ffmpeg version 0.8.3-4:0.8.3-0ubuntu0.12.04.1, Copyright (c) 2000-2012 the Libav developers
  built on Sep 21 2012 18:38:36 with gcc 4.6.3
*** THIS PROGRAM IS DEPRECATED ***
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
[video4linux2 @ 0x1f4ca20] Estimating duration from bitrate, this may be inaccurate
Input #0, video4linux2, from '/dev/video1':
  Duration: N/A, start: 20714.773053, bitrate: N/A
    Stream #0.0: Video: mjpeg, yuvj422p, 1280x720, -5 kb/s, 30 tbr, 1000k tbn, 30 tbc
[video4linux2 @ 0x1f4d9a0] Estimating duration from bitrate, this may be inaccurate
Input #1, video4linux2, from '/dev/video2':
  Duration: N/A, start: 20715.074053, bitrate: N/A
    Stream #1.0: Video: mjpeg, yuvj422p, 1280x720, -5 kb/s, 30 tbr, 1000k tbn, 30 tbc
Incompatible pixel format 'yuvj422p' for codec 'mpeg4', auto-selecting format 'yuv420p'
[buffer @ 0x1f4d480] w:1280 h:720 pixfmt:yuvj422p
Output pad "default" for the filter "src" of type "buffer" not connected to any destination
Error opening filters!