ffmpeg: cómo agregar varios flujos de subtítulos

Intenté agregar 2 subtítulos .srt a un video, pero el archivo resultante solo tiene el primer subtítulo. Este es el comando que utilicé:

ffmpeg -i $movie.mov -i $sub_en.srt -i $sub_de.srt \ -c:v copy -c:a copy \ -c:s:0 srt -metadata:s:s:0 language=eng \ -c:s:1 srt -metadata:s:s:1 language=ger \ $output.mkv 

El resultado completo a continuación también muestra que solo codifica 1 subtítulo (reemplacé los nombres largos de archivo con las variables utilizadas anteriormente para claridad):

ffmpeg version 3.3.2-static http://johnvansickle.com/ffmpeg/ Copyright (c) 2000-2017 the FFmpeg developers built with gcc 5.4.1 (Debian 5.4.1-11) 20170519 configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-5 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gray --enable-libass --enable-libfreetype --enable-libfribidi --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libzimg libavutil 55. 58.100 / 55. 58.100 libavcodec 57. 89.100 / 57. 89.100 libavformat 57. 71.100 / 57. 71.100 libavdevice 57. 6.100 / 57. 6.100 libavfilter 6. 82.100 / 6. 82.100 libswscale 4. 6.100 / 4. 6.100 libswresample 2. 7.100 / 2. 7.100 libpostproc 54. 5.100 / 54. 5.100 Input #0, mov,mp4,m4a,3gp,3g2,mj2, from "$movie.mov": Metadata: major_brand : qt minor_version : 0 compatible_brands: qt creation_time : 2017-08-23T15:33:19.000000Z Duration: 00:56:34.92, start: 0.000000, bitrate: 20807 kb/s Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 20546 kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc (default) Metadata: creation_time : 2017-08-23T15:33:20.000000Z handler_name : Core Media Data Handler encoder : H.264 timecode : 01:00:00:00 Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 255 kb/s (default) Metadata: creation_time : 2017-08-23T15:33:20.000000Z handler_name : Core Media Data Handler Stream #0:2(und): Data: none (tmcd / 0x64636D74) (default) Metadata: creation_time : 2017-08-23T15:33:20.000000Z handler_name : Core Media Data Handler timecode : 01:00:00:00 Input #1, srt, from "$sub_en.srt": Duration: N/A, bitrate: N/A Stream #1:0: Subtitle: subrip Input #2, srt, from "$sub_de.srt": Duration: N/A, bitrate: N/A Stream #2:0: Subtitle: subrip Output #0, matroska, to "$output.mkv": Metadata: major_brand : qt minor_version : 0 compatible_brands: qt encoder : Lavf57.71.100 Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 20546 kb/s, 25 fps, 25 tbr, 1k tbn, 25 tbc (default) Metadata: creation_time : 2017-08-23T15:33:20.000000Z timecode : 01:00:00:00 handler_name : Core Media Data Handler encoder : H.264 Stream #0:1(ger): Audio: aac (LC) ([255][0][0][0] / 0x00FF), 48000 Hz, stereo, fltp, 255 kb/s (default) Metadata: creation_time : 2017-08-23T15:33:20.000000Z handler_name : Core Media Data Handler Stream #0:2: Subtitle: subrip (srt) Metadata: encoder : Lavc57.89.100 srt Stream mapping: Stream #0:0 -> #0:0 (copy) Stream #0:1 -> #0:1 (copy) Stream #1:0 -> #0:2 (subrip (srt) -> subrip (srt)) Press [q] to stop, [?] for help frame=20372 fps=236 q=-1.0 Lsize= 2091417kB time=00:13:35.78 bitrate=21001.7kbits/s speed=9.45x video:2065455kB audio:25494kB subtitle:8kB other streams:0kB global headers:0kB muxing overhead: 0.022014% Exiting normally, received signal 2. 

Sé que en este caso particular puedo usar mkvmerge, pero no funciona para subtítulos .mp4 mov_text (.ttxt), y de todos modos me gustaría entender qué está mal con mi comando ffmpeg.