This system was really cool. It was frustrating though when 90% of people never bothered or you couldn't ever get them to understand how it works. But with my irl friends/fam we used it all the time. When we had it, it made me think of Chrono Trigger and how different class's could create different "Dual-Techs" or "Tri-Techs". It made me wish we could have a elaborate system like that for doing different kind of Battle Regimens. They could do this in XIV Depending on the combination of players with you, you could do certain LBs. It could be done ala skill chains FFXI. Or we could get LB actions of sorts. And so long as the right class is in your group and has LB bar(the individual lb bar w/e form it is when it comes out) then the skill lights up on your hotbar.
Both players would need to press it. The first one that does, initiates a animation indicating they are waiting for the other player. That way it's engaging, you see that pld winding up for a dual LB, so you join them for a big boooom. They'd clearly have to be very careful not to make one more significantly more powerful then the other. Sense the current LB isn't powerful anyways, I don't think we'd have to worry about that. But it could be used to fix some situations, like when your in a party of 4, and no melee lb. But like WHM + BARD = Holy Eagle Eye Shot. SCH+WHM = CureIII+Sucor spell of some sort. I'm sure SE already has plans for some kind of DPS LB anyways for BARDs with the individual lb thing. But anyhow a well thought out, engaging LB Dual/Triple tech system would be very fun, if it was done right. And was properly taught to players in game. I think LBs should already be way more available then they already are. Completing boss's 90% of the time with just one bar lb, and not having it most of a dungeon run seems kind of sad to me. You could say its balanced, but I think its a little under tuned atm, imo.
Back on topic, I think being able to disable certain parts of the mob would be great. Unfortunately it was kinda like the battle regimen system in that lots of people couldn't be bothered with figuring it out. If they re-introduced it, they would have to make it easier for the masses, or at least easier to learn. Also they would need several quests to teach people how to use the system. Not things like our class/job quests where you can do anything to win, but you MUST break this part of the monster in order to continue, and have a very clear explanation of what this just did to the mob. And how important it is to attempt this on monsters outside the quest. Hell have part of the quest take you into Ifrit HM to attempt it, and if you succeed quest complete.