Play two videos in sync frame by frame with different starting positions

I want to compare two videos (with different resolutions) and want to go though both videos frame by frame (forward and backward) in sync. I managed to do so using

mpv somevideo720.mkv --external-file=somevideo1080.mkv --lavfi-complex='[vid1] scale=960x540 [vid1_scale]; [vid2] scale=960x540 [vid2_scale]; [vid1_scale][vid2_scale] hstack [vo]'

(compare [here](https://github.com/mpv-player/mpv/issues/3854#issuecomment-571424357)) However I also want to set up two different starting positions of the videos. How can I do this without modifying the videos itself? The best solution would be if it would be possible to open both videos, navigate in both videos to the desired starting position, then go into "sync mode"...