FFmpeg - Overlay one video onto another video?

I understand that this is a very open ended question. I have done some initial reading into FFmpeg, but now require some guidance.

Problem

• I have a video input.mov.
• I would like to overlay another video on top of overlay.wov.
• The result should be a single video (output.mov).

Notes

• Done some initial reading into FFmpeg and read this question.

Thanks - C.

Edits

1. Backend is Go/Ruby. Open to using a new language.
2. The audio from the first video should be kept.
3. Setting the interval at which the overlay starts would be great.

Current Solution

ffmpeg -i input.mov -i overlay.mov -filter_complex "[0:0][1:0]overlay[out]" -shortest -map [out] -map 0:1 -pix_fmt yuv420p -c:a copy -c:v libx264 -crf 18  output.mov

This nearly works, however:

• Overlay is cut short even though the two videos (input.mov & overlay.mov) are the same length.
• I cannot start the overlay at any interval apart from 0:00.

If you just want a ffmpeg command, try

ffmpeg -i input.mov -i overlay.mov \
-filter_complex "[1:v]setpts=PTS-10/TB[a]; \
                 [0:v][a]overlay=enable=gte(t\,5):shortest=1[out]" \
-map [out] -map 0:a \
-c:v libx264 -crf 18 -pix_fmt yuv420p \
-c:a copy \
output.mov

This starts the overlay at 5 seconds with the overlaid video start point being 00:15.

setpts=PTS-10/TB is setpts=PTS+(overlay_delay-video_trim_in)/TB

overlay=enable=gte(t\,5) is overlay=enable=gte(t\,overlay_delay)