quite a few problems with that.
First, you pretty much force the player into gimping themselves by taking such characters without forcing them to take all of them pretty much. Or else they'll have as poor a spell selection as Remi, who makes up for it by her great stats and great buff. If they have great stats and great buffs, then their combo attacks will either be op, or redundant to their solo moves.
I could go on but thats the important one. The game as a whole isn't really made for team-based characters, so you can't just slap one in with it being unbalanced. They'd work out ifne like that if there were other similar teamwork type deals, but the closest thing you got is spell, which itself is a waste of 100 sp unless you are using all 3 characters, in which case it's pretty much meh. But she has 4 other spells to go with it, along with amazing stats.
How about having something a bit similar like
spell? Something like..well I'm gonna just be a bit random with this, but hopefully you understand.
Merlin is a Debuff type character, and Lyrica is an Attack type character. Their dual attack can be used separately. For Lyrica, its an attack, and for Merlin, its a debuff. If both are in the party when used, both of their Active Gauge's are reduced down to a certain point(30% after use, for example), regardless of how high the Gauge is filled(If lower than 30%, then no change is made), and the SP cost is used from both of them. If both are in the active party at the time it is used, the Spell is both an Attack and Debuff type spell. Lunasa would be a Tank type(possibly) with the ability to buff. The Triple Team attack would be similar to a dual attack only reducing the gauge to 0%, dealing damage to the enemy and debuffing them, and also buffing your party. If Merlin uses alone, its mainly a debuff(A bit of a heavy one), if Lyrica its mainly an attack, and if Lunasa uses it alone its just a buff. For all dual-spells and the trio one, the attack/buff/debuff rates are increased.
This isn't to say their Solo skill would mainly be Attack-Buff-Debuff only either. Could be a mixture of both like other skills are. If you think about it though, the above makes it possible for the characters to be used both separately and as a team.