I don't think anybody can tell you what to chose, because there are WAY too many possible 2-character combination.
What you could do is write down the most important abilities/features of a class and pick your 2 best.

Think of:
- armor (chose high armor if you want pvp survivability)
- healing (do you want 1 good DPS and 1 good healer? or just 2 dps with average healing)
- resurrection and wipe recovery (e.g. reincarnation of shaman class)
- dps (do you want 1 or 2 classes with decent/high dps?)
- buffs (can they stack?)
- pvp and pve viability

[edit] But 2 shamans would be my choice too (2x wipe reincarnation, 2x healing, stackable totems, decent DPS) but I would make them Elemental. It's probably the easier class to start with anyway and if one of them dies, the other can do just the same actions.