This is actually isn't very hard to accomplish through eventing and very minor scripting. All you need to do is make a script call to check if a certain party member is currently active in battle, and that script call is just something like this:
$game_party.battle_members.include?($game_actors[x])
So you can effectively set up a conditional branch on the troop page to check if certain actors are active in battle. If, for example, you wanted to check if both actors 1 and actors 2 were in battle together, and if so wanted to increase their potential support points each turn they're out there together and not dead, you could have a conditional branch like this:
And so they'll always gain 1 point every turn they're in battle together. Then, if you wanted to check for when they finally have enough points to get to Support C, all you have to do is make another troop event like this:
(MINOR EDIT: it should span turn, not battle. MY BAD)
And that's it! That's how I would do it at least. Of course that doesn't do anything by itself, so I'd make a common event for Support C for Actors 1 and 2 that gives them a boosting state, like so:
And then every time they're in battle together, they'll always get that boost, and whenever either of them is dead or not in battle anymore, they lose it. It looks like it might be intimidating and a lot of work, but you only need to make the troop events once and then just copy the page over to every other troop. You don't really need an actual script beyond the minor script call. And unless you're having a FE number of characters (like 30+ playable characters), it won't take that long for you to get a few supports set up. It took me like 10 minutes tops to set up that explanation Actor 1+2 Support C.
I'm not saying this is the best way to do it or the only way to do it. It's just how I would go about getting it done.
*edit* Keep in mind this is just how I would implement a straight FE type of support system without any scripts whatsoever. If you want to use it to acquire combo skills with Victor's script, the process should be very similar. In fact it's a lot simpler: rather than requiring a common event or a state, all you do is have the actors acquire the combo skill after the support conversation, then turn the switch on to disable the convo from happening again. You don't need even to do the steps in the third and fourth image I posted.