What problem is this trying to solve? Well, the one about finding galleries featuring a story with a single pairing pretty difficult. It is impossible to define a search for character A, character B and no other characters (plus, such a search would return galleries with let's say story 1 A+tentacles, story 2 B solo action)

If one wants to find a specific pairing of characters A and B, currently there is no easy way of getting ONLY those results. Search for A+B will also return galleries where A has sex with C and B with D, groups, and so on. Trying to do -group will exclude galleries which have multiple stories (ex. first is A+B, second is A+B+C). We don't have a tag to indicate galleries with multiple unrelated stories. Sole tags are useless as they can are excluded with a single rogue image: Consider those two examples:
* http://ehentaihip.com/g/1068963/0b98c4e54e/ (regular size)
* http://ehentaihip.com/g/1094313/e9f5ce15cc/ (140 pages)
Both feature a single couple having sex, except each has a single page showing another pairing. Still, that single other pairing means sole tags are invalid.

Anyway, there is no easy way to find galleries of A+B through exclusions. So the only other way is to find it through inclusion of a new type of tag - the one presented here. With it, searching for A+B+couple should return galleries in which there is at least some (presence-filtered) solo action between a couple, even if there are other stories/groupings. It is still not ideal as it will also return results A+E, B+F, and C+D couple but there is nothing better short of couple-specific tags and that is not going to fly... . Now, if we could also implement the 'no other characters beside those specified' option it would be perfect, but that goes way beyond tagging.

The point is this should improve searches for people wanting to get A+B only action.

I hope not to hear 'this tag would need to be added to too many galleries', since we did it (well, are doing...) with sole tags - this one shouldn't be worse.