Depends who you care more about.
Invokers are limited to “dragon” stats.
Conquerer has 3% dragon attack and 2% dragon HP which is all the invoker will benefit from.
Rosheen has 5% dragon attack and 10% dragon HP and 10% rage but has 10% sorcerer HP that would be wasted.
Conquerer 13 attack and 12 HP
Rosheen 5% attack and 20% HP and 10% Rage.
Overall to get the most benefit for each dragon I would say Morak-Rosheen and FafNyr-Conquerer until you can get another.
Since Fafnyr doesn’t have rage regeneration over time I don’t think he would benefit from having Rosheen as much.
That’s my initially thought anyway