$ ffmpeg -i song_1.mp3 -i watermark.mp3 -filter_complex "amerge,pan=stereo|c0<c0+c2|c1<c1+c3" output.mp3
ffmpeg version git-2013-11-09-64f4816 Copyright (c) 2000-2013 the FFmpeg developers
  built on Nov  9 2013 19:16:51 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --prefix=/home/ussher/ffmpeg_build --extra-cflags=-I/home/ussher/ffmpeg_build/include --extra-ldflags=-L/home/ussher/ffmpeg_build/lib --bindir=/home/ussher/bin --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-nonfree --enable-x11grab
  libavutil      52. 52.100 / 52. 52.100
  libavcodec     55. 41.100 / 55. 41.100
  libavformat    55. 21.100 / 55. 21.100
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.102 /  3. 90.102
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
[mp3 @ 0x2210240] Estimating duration from bitrate, this may be inaccurate
Input #0, mp3, from 'song_1.mp3':
  Duration: 00:00:20.06, start: 0.000000, bitrate: 64 kb/s
    Stream #0:0: Audio: mp3, 24000 Hz, stereo, s16p, 64 kb/s
Input #1, mp3, from 'watermark.mp3':
  Duration: 00:00:11.57, start: 0.000000, bitrate: 128 kb/s
    Stream #1:0: Audio: mp3, 24000 Hz, mono, s16p, 128 kb/s
[Parsed_amerge_0 @ 0x22112c0] No channel layout for input 1
Output #0, mp3, to 'output.mp3':
  Metadata:
    TSSE            : Lavf55.21.100
    Stream #0:0: Audio: mp3 (libmp3lame), 24000 Hz, stereo, s16p (default)
Stream mapping:
  Stream #0:0 (mp3) -> amerge:in0
  Stream #1:0 (mp3) -> amerge:in1
  pan -> Stream #0:0 (libmp3lame)
Press [q] to stop, [?] for help
[mp3 @ 0x2210c40] overread, skip -5 enddists: -1 -1
[mp3 @ 0x2210c40] overread, skip -6 enddists: -2 -2
[mp3 @ 0x2210c40] overread, skip -7 enddists: -5 -5
[mp3 @ 0x2210c40] overread, skip -5 enddists: -1 -1
[mp3 @ 0x2210c40] overread, skip -7 enddists: -4 -4
[mp3 @ 0x2210c40] overread, skip -6 enddists: -4 -4
[mp3 @ 0x2210c40] overread, skip -6 enddists: -2 -2
[libmp3lame @ 0x2252f00] Trying to remove 576 samples, but the queue is empty
size=      91kB time=00:00:11.54 bitrate=  64.4kbits/s
video:0kB audio:91kB subtitle:0 global headers:0kB muxing overhead 0.229684%