I believe one mod indeed would have to handle the whole thing but to get to the actual NPC from the other mod is not that hard: getFormFromFile.
I just think this is an unexplored scenario in any other mods that can be fun: player to be the matchmaker for two followers with their story evolving while they follow.
Actually maybe more interesting would be just a like/hate meter between the two of them instead of a quest based story.
Different events may move the meter and the relation could be unsymmetrical between the two of them. Events like:
- player giving better items to one of them
- admiring each-other for the fighting skills, kills, execution of commands like lockpick/attack
- attacking the attacker of the other when one is in difficulty - and you shouldn't script that to happen, only count it if is the result of chance
- stepping on each-other toes because pure vanilla situation: blocking path, dealing a killing blow to an enemy the other did most of the damage
- etc
On different levels they would have different reactions:
- comments
- skill/damage bonuses/penalties when they are together or apart
- refusing orders if they both follow or if the other isn't around when they like eachother
- "accidentally" friendly fire during combat
- refusing to fight
Release the tension:
- player orders them to stop that, resulting in a penalty to the relationship with player
- a brawl if the relation is really low then it will reset
- a hug/kiss animation or something if is high