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

ffmpeg version N-67227-g39680ca Copyright (c) 2000-2014 the FFmpeg developers
  built on Oct 27 2014 22:10:36 with gcc 4.9.1 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfi
g --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-lib
opencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinge
r --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --
enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
libx265 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      54. 11.100 / 54. 11.100
  libavcodec     56. 10.100 / 56. 10.100
  libavformat    56. 11.100 / 56. 11.100
  libavdevice    56.  2.100 / 56.  2.100
  libavfilter     5.  2.100 /  5.  2.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Guessed Channel Layout for  Input Stream #0.1 : mono
Input #0, rtsp, from 'rtsp://ip/videoSub':
  Metadata:
    title           : IP Camera Video
    comment         : videoSub
  Duration: N/A, start: 0.000000, bitrate: 64 kb/s
    Stream #0:0: Video: h264 (Baseline), yuv420p, 320x240, 14.99 tbr, 90k tbn, 180k tbc
    Stream #0:1: Audio: pcm_mulaw, 8000 Hz, 1 channels, s16, 64 kb/s
Guessed Channel Layout for  Input Stream #1.1 : mono
Input #1, rtsp, from 'rtsp://ip/videoSub':
  Metadata:
    title           : IP Camera Video
    comment         : videoSub
  Duration: N/A, start: 0.000000, bitrate: 64 kb/s
    Stream #1:0: Video: h264 (Baseline), yuv420p, 320x240, 90k tbr, 90k tbn, 180k tbc
    Stream #1:1: Audio: pcm_mulaw, 8000 Hz, 1 channels, s16, 64 kb/s
[libx264 @ 00000000054b4580] using SAR=1/1
[libx264 @ 00000000054b4580] using cpu capabilities: MMX2 SSE2Fast LZCNT
[libx264 @ 00000000054b4580] profile High, level 2.1
Output #0, mpegts, to 'udp://239.101.101.1:1234':
  Metadata:
    title           : IP Camera Video
    comment         : videoSub
    encoder         : Lavf56.11.100
    Stream #0:0: Video: h264 (libx264), yuv420p, 320x480 [SAR 1:1 DAR 2:3], q=-1--1, 25 fps, 90k tbn, 25 tbc (default)
    Metadata:
      encoder         : Lavc56.10.100 libx264
    Stream #0:1: Audio: mp2, 16000 Hz, mono, s16, 160 kb/s
    Metadata:
      encoder         : Lavc56.10.100 mp2
Stream mapping:
  Stream #0:0 (h264) -> setpts (graph 0)
  Stream #1:0 (h264) -> setpts (graph 0)
  overlay (graph 0) -> Stream #0:0 (libx264)
  Stream #0:1 -> #0:1 (pcm_mulaw (native) -> mp2 (native))
Press [q] to stop, [?] for help
[NULL @ 00000000003017a0] RTP: missed 13 packets00:00:04.62 bitrate= 131.5kbits/s
[h264 @ 0000000008c24100] Cannot use next picture in error concealment95.6kbits/s
[h264 @ 0000000008c24100] concealing 146 DC, 146 AC, 146 MV errors in P frame
[NULL @ 00000000003017a0] RTP: missed 11 packets00:00:04.62 bitrate= 366.7kbits/s
[h264 @ 0000000008c24100] concealing 132 DC, 132 AC, 132 MV errors in I frame
[NULL @ 0000000002bfb400] RTP: missed 1 packets=00:00:12.18 bitrate= 370.5kbits/s
[NULL @ 0000000002bfb400] RTP: missed 1 packets=00:00:13.67 bitrate= 361.1kbits/s
[NULL @ 0000000002bfb400] RTP: missed 2 packets=00:00:21.65 bitrate= 356.9kbits/s
[NULL @ 0000000002bfb400] RTP: missed 2 packets=00:00:30.28 bitrate= 352.1kbits/s
[NULL @ 0000000002bfb400] RTP: missed 2 packets=00:00:38.75 bitrate= 339.3kbits/s
[NULL @ 0000000002bfb400] RTP: missed 2 packets=00:00:47.80 bitrate= 340.3kbits/s
[NULL @ 0000000002bfb400] RTP: missed 2 packets=00:00:56.84 bitrate= 340.0kbits/s
frame= 1485 fps= 26 q=-1.0 Lsize=    2400kB time=00:00:59.32 bitrate= 331.4kbits/s
video:919kB audio:1108kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 18.373344%
[libx264 @ 00000000054b4580] frame I:6     Avg QP:17.88  size: 44772
[libx264 @ 00000000054b4580] frame P:384   Avg QP:20.21  size:  1571
[libx264 @ 00000000054b4580] frame B:1095  Avg QP:30.25  size:    63
[libx264 @ 00000000054b4580] consecutive B-frames:  1.5%  0.3%  0.4% 97.8%
[libx264 @ 00000000054b4580] mb I  I16..4: 13.7% 13.4% 72.9%
[libx264 @ 00000000054b4580] mb P  I16..4:  0.5%  0.1%  0.1%  P16..4: 44.2%  2.2%  3.3%  0.0%  0.0%    skip:49.6%
[libx264 @ 00000000054b4580] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8: 10.1%  0.0%  0.0%  direct: 0.0%  skip:89.9%  L0:33
.2% L1:66.7% BI: 0.0%
[libx264 @ 00000000054b4580] 8x8 transform intra:13.8% inter:81.8%
[libx264 @ 00000000054b4580] coded y,uvDC,uvAC intra: 67.1% 94.7% 84.5% inter: 1.3% 11.3% 5.1%
[libx264 @ 00000000054b4580] i16 v,h,dc,p:  4% 12%  2% 82%
[libx264 @ 00000000054b4580] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 22% 17% 13% 10%  4%  6% 12%  7% 10%
[libx264 @ 00000000054b4580] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 17% 16% 14% 12%  5%  5% 13%  7% 12%
[libx264 @ 00000000054b4580] i8c dc,h,v,p: 58% 15% 17% 10%
[libx264 @ 00000000054b4580] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 00000000054b4580] ref P L0: 71.8%  1.9% 18.3%  8.0%
[libx264 @ 00000000054b4580] ref B L0: 96.3%  3.4%  0.3%
[libx264 @ 00000000054b4580] ref B L1: 98.0%  2.0%
[libx264 @ 00000000054b4580] kb/s:126.78
Received signal 2: terminating.