ffmpeg: concatenate videos of different frame rates? [duplicate] - ffmpeg

Question

I have two input videos that I am concatenating using the FFmpeg concat demuxer:
ffmpeg -f concat safe '0' -i /path/to/file.txt -c copy /path/to/output.mp4
The file.txt contains the two file paths as follows:
ffconcat version 1.0
file /path/to/input/file1.mp4
file /path/to/input/file2.mp4
Unfortunately, the concatenated video has two issues:
The duration of the first input video is 4 seconds. In the output video, the last frame freezes for several seconds before the second input video starts.
The playback speed of the second input video is slowed down so that instead of being 7 seconds long, the video plays for around 14 or 15 seconds.
I have tried specifying the duration of each video in the .txt file, as specified by the FFmpeg docs, but this doesn't seem to make a difference to the output.
I'm a bit of a newbie to FFmpeg, so any help is greatly appreciated!
Edit - The properties of each input video as given by FFprobe:
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'tmp/video/16382802.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.25.100
Duration: 00:00:04.43, start: 0.000000, bitrate: 644 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 360x640, 640 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
Metadata:
handler_name : VideoHandler
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'tmp/video/16382805.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.25.100
Duration: 00:00:07.31, start: 0.023220, bitrate: 836 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 360x640, 669 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, 5.1, fltp, 158 kb/s (default)
Metadata:
handler_name : SoundHandler
Edit 2 - Stack trace when rewrapping the video (audio is lost):
ffmpeg version 3.0.1 Copyright (c) 2000-2016 the FFmpeg developers
built with Apple LLVM version 7.3.0 (clang-703.0.31)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.0.1 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-libfreetype --enable-ffplay --enable-libfdk-aac --enable-frei0r --enable-nonfree --enable-vda
libavutil 55. 17.103 / 55. 17.103
libavcodec 57. 24.102 / 57. 24.102
libavformat 57. 25.100 / 57. 25.100
libavdevice 57. 0.101 / 57. 0.101
libavfilter 6. 31.100 / 6. 31.100
libavresample 3. 0. 0 / 3. 0. 0
libswscale 4. 0.100 / 4. 0.100
libswresample 2. 0.101 / 2. 0.101
libpostproc 54. 0.100 / 54. 0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'https://s3-eu-west-1.amazonaws.com/stories.snaplytics.io/fTYbaN78DBVEQI0js0ydhNw/d3ef9a13-454c-4015-8412-cbd890e70e24.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.25.100
Duration: 00:00:07.31, start: 0.023220, bitrate: 746 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 360x640, 669 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 67 kb/s (default)
Metadata:
handler_name : SoundHandler
Input #1, lavfi, from 'anullsrc':
Duration: N/A, start: 0.000000, bitrate: 705 kb/s
Stream #1:0: Audio: pcm_u8, 44100 Hz, stereo, u8, 705 kb/s
Output #0, mp4, to '/Users/barnabytaylor/Documents/fanbytes-dashboard/tmp/video/16382805.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.25.100
Stream #0:0(und): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 360x640, q=2-31, 669 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 30k tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1: Audio: aac (LC) ([64][0][0][0] / 0x0040), 44100 Hz, 5.1, fltp, 341 kb/s
Metadata:
encoder : Lavc57.24.102 aac
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #1:0 -> #0:1 (pcm_u8 (native) -> aac (native))
Press [q] to stop, [?] for help
frame= 219 fps= 57 q=-1.0 Lsize= 98706kB time=00:00:07.31 bitrate=110550.2kbits/s speed=1.91x
video:598kB audio:6kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 16257.420898%
[aac # 0x7ffbe2806e00] Qavg: 65536.000

Answer 1

The first video has a smaller timebase (and no audio track). Rewrap it like this and then concat:
ffmpeg -i 16382802.mp4 -f lavfi -i anullsrc -c:v copy -video_track_timescale 30k -c:a aac -ac 6 -ar 44100 -shortest new.mp4