Create Alpha channel-Videos (obs stinger compatible) with ffmpeg and 2 files
The alphaextract/alphamerge filters are meant for this.

Since your alpha reference is already greyscale, we only need the alphamerge filter

ffmpeg -i main.mp4 -i alpha.mp4 -filter_complex [0][1]alphamerge,format=yuva420p -c:v libvpx -auto-alt-ref 0 out.webm