Well, Id say that the best duo is anything with a Cleric. In that sense, Priest is the better option (good healing/buff capability) for early game. Assuming you want to play casually and dont bother with min-maxing, tis even possible to pick a DPS path later (like Monk, Paladin, Sadhu), just to speed up things a bit.
Now the counterpart, you could be a real gentleman and let you lady play with this one. Just saying~ lol but for real, if youre into melee the obvious choice is Swordsman, so anything goes (Highlander/Barb/Corsair, Hoplite/Cata/Dragoon, Barb/Corsair/Doppel for example) if, as I said, you want to play casually, but just make sure to have at least one level of Peltasta for mobbing. It helps a lot.
BUT! If youre into the hogwarts wannabes, going DPS (loool) is better, because youre (hopefully) pairing with a Cleric and picking support paths will only slow you down. Pyro/Linker is really popular, as much as Wiz C3/Ele C3.
Last but not least, for some REAL deeps you can go Archer. If you dont want to think that much, just go QS3/Falconer/Cannoneer. Literally BOOM, youll have good sustained auto-attack and amazing AoE dmg. If youre just looking for having fun, Id recommend Ranger/Sapper/Scout.
I think thats it.
Anyways, I hope both of you enjoy your journey together and have lots of fun!