If I remember correctly, IS said they were planning a duo unit for every single main character.
So you can probably expect one for Ike, but one for Elincia will depend. Does IS consider Elincia a main character? I don't think she was in PoR, and in RD, she quickly got overshadowed by Ike and Micaiah.
So unless she's the futur duo partner for Ike-- which could be unlikely there are many characters that could be with Ike like Soren, Mist, Greil, Titania, etc-- I don't know how else she will be a duo unit.