How to compare two different previews with A/B comparison mode?

How is the A/B comparison mode used to compare two different previews? All I ever see is that the unprocessed video is A, and the selected preview is B.

I read somewhere – and cannot find it now, and am sure it was in the online documentation – that A and B can be independently assigned to different previews. Obviously, the two previews should cover the exact same timespan, but how to make two previews starting at exactly the same time eludes me as well.

Thanks,
- Eric

Bumping this because I have the same issue using 5.5.1 and can not figure out how to assign A to a different preview.