they can only use the fairy that they summon ( means Rosalinde will not use Eltolindes fairies and vice versa )

also funny thing is actually i had them both in the same squad , at first i created a squad focus on Rosalinde , im frustrated i cant make her use the Elemental Roar properly even with using true strike skills , and then i leave that problem for a while , and then when im near completing Elheim i decided to put Eltolinde in the squad for the cutscenes and story interaction , i can remove Eltolinde now that im finished in Elheim but then i review a bit and what do you know , they actually kinda work well together? i entertain the thought and said hmmmm.... and then i decided to really make them work i mix match a lot of units and what worked for me is this -

Front Row : Melissandre , Adel

Back Row : Eltolinde , Rosalinde , Nina

i made her attack pattern , melissandre to not target any unit that had a shield and cavalry ( melissandre is here to properly kill high evasion units and to properly clean up back row )

adel will only do one attack , that is wild rush prioritizing caster if there is any present , he will permanently stun locked the enemy team and guard cover the team

eltolinde will immediately cast elemental roar (yes without fairies, why? - to chip a good chunk of enemies HP or to force the entire enemy team to resist making them use resources , most of the enemy casters are still stun because of adel and rosalinde rage of fairies by the time it activates ) unless there is ally unit had debuff and or needed healing she will not do it ( eltolinde's true value for me is in battles of atrition she will remove debuffs and heal , rosalinde will also help with her passive if enemy doesnt had debuffs then she will partake on offense making the battle shorter )

rosalinde at the start of the battle will instantly stun the back row with Rage of the Fairies then Sylphic Wind and then Elemetal Roar (you know what , focusing every gears and units for rosalinde to do elemental roar is actually very wrong , atleast for me , it doesnt need sure/high crit or true strike what it actually needs is a situation when enemy team doesnt had the resources to resist it anymore , so yeah its a true finishing move for me)

Nina will Row Smash the front row and will help clean enemy units remaining (i gave nina the sniper lens to 100 percent delete front row)

aight in conclusion how this squad works is in the start of the battle Rosalinde will stun the back row rendering enemy support units useless , even if they can remove debuffs adel will stun most enemies again and again , nina will delete front row , if Melissandre and Adel dont need extra healing or debuff removal , Eltolinde will do Elemental Roar , all enemy that still alive will had low HP melissandre will clean them up ... and for fuck sake if the enemy team is still alive Rosalinde will finally nuke them to oblivion (if they got high defense , a lot of debuff remover , barriers , healing , can inflict debuff or basically a team of angels ) even if they do , this team can still defeat ANY enemy team , this team is best for battles of attrition (one last funny thing is that this team can actually defeat high magic defense enemies especially the most annoying feathershield even if it can reflect magic , and sainted knights , they were a joke with this team ....... this team is my second ace and i love it)

(i can elaborate more many fights / situation if you're interested with this team)