Control video offset individually to try to manually sync
This will be tricky in your lovely minimal UI, but it would be tremendously helpful if there were a way to offset individual videos to try to achieve sync between two versions that don't have identical runtimes.
This could be approximate with a scrubber, or some kind of frame vs. frame lineup, or even cooler if it guesses based on some audio or image analysis.... But without a way to offset at all, the synchronized scrubbing doesn't get you very far if the sources aren't perfectly matched to begin with.