How to concatenate 2 MP4 videos to output of certain bitrate?
Step 4 (first attempt) will work if you re-encode the 2nd video like the first (without the filters).

What's happening is that besides the timescale difference between CC's output and ffmpeg's (which will only result in timing errors), x264 is producing a High profile stream with a different parameter set than CC's encoder. Re-encode the 2nd video with ffmpeg and your files should be matching. Alternatively, you can speed up in CC and export the 1st video.