>ffmpeg -y -f video4linux2 -vcodec mjpeg -s 1280x720 -r 30 -i /dev/video1 -f video4linux2 -vcodec mjpeg -s 1280x720 -r 30 -i /dev/video2 -map 0 -map 1 -vcodec copy -an -ar 44100 output.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 @ 0xd07a20] Estimating duration from bitrate, this may be inaccurate
Input #0, video4linux2, from '/dev/video1':
  Duration: N/A, start: 16993.324787, bitrate: N/A
    Stream #0.0: Video: mjpeg, yuvj422p, 1280x720, -5 kb/s, 30 tbr, 1000k tbn, 30 tbc
[video4linux2 @ 0xd089a0] Estimating duration from bitrate, this may be inaccurate
Input #1, video4linux2, from '/dev/video2':
  Duration: N/A, start: 16993.653784, bitrate: N/A
    Stream #1.0: Video: mjpeg, yuvj422p, 1280x720, -5 kb/s, 30 tbr, 1000k tbn, 30 tbc
Number of stream maps must match number of output streams