In a private league I'm playing we're planning an event where there's two (equal-sized) groups of teams each of which should play each of the other group's teams (round-robin-style), but never one from the own group.

At the moment, we'll schedule the games manually and use a ladder-style format but it would be nice if you could set this up as a tournament option.

As this is a very special case, another approach would be some sort of free-style tournament-planning option which would allow the admin/creator of a competition to just manually add any pairing they want to a match-day. That way, it would be possible to realize the original idea but also any other crazy competition ideas other people can come up with.