I'm not sure where or how this can be achieved - However, when I am putting edits together in my editorial software (primarily flame), I am constantly bouncing between a side x side view (which is currently available in ftrack review), a split screen (generally an angle split, but horizontal or vertical would be nice) and a difference (black&white difference matte between the two versions).
Being able to compare 2 different versions in these ways would be extremely valuable in the commercial and short form online markets.
Maybe this would have more potential to show up in CineSync?