For just two characters any two will work, but the follower would ideally be a non-melee character; hunter, mage, lock, druid, priest, shaman. But for a duo two of the same type is not always optimal, I have duoed everything on that list as pairs. So far in order of effectiveness and survivability: (not done priest yet)

Hunter > Lock > Shaman > Mage > Druid

Hunter - Good pets for tanking; very good agro management if you follow the BM tree; plus agro shedding at 30+ with FD; light on mana (sting+auto shot); ability to take on significantly higher mobs (at 31 I'm killing 37s with some ease); and reasonable CC.

Lock - Same as above really; squishy which is bad but soul stone mitigates that a bit; less burst damage so less chance of gaining agro off the pets; no need to face the target for a lot of the time; less LOS issues; fears when used well can really help you.

Shaman - Totems... ; Good for utility; lacks damage output of a Hunter/Lock (note: For leveling I'm sure a well geared elemental is a great thing to have/play but from my tests a low level pair of shaman (any spec) just doesn't do the damage that a hunter does); more survivable than hunter/lock with both a self res and a party res; nice travel form at 20; water walking yay; Don't get me wrong here I love my shamans but I just don't find them as a duo to be that powerful.

Mage - Good if you can kill the mobs quickly if not or your CC fails/resists bad... but then I'm not a big fan of mages any way so it might just be me being crap at them. So good for damage (and portals) crap for anything else.

Druids - I can't find a happy medium for druids, the main is okay in tank mode, the follower is okay too, but what does she do? In tank mode the main virtually never needs healing, and the damage output of a low level caster druid is pointless. Tank knocks a third of the health off the mob per hit, caster does about a tenth of that if she crits. :S

Anyway thats how I see it.

Essentially for a duo what you're looking at is a solo player who gets half XP per mob, but has extra utility from the follower. In the case of my hunter duo that extra utility is a 2nd pet and heaps of damage. But if you want to break things up you could go with a tank (warrior/pally/bear) and DPS/Healer. You play the tank as a main and the DPS/Healer gives you the extra damage you need to burn through the mobs.

Make no mistake you will burn through the mobs, as a duo unless you have some good friends who understand what you're doing, or are already in a guild thats cool with the idea, you're going to struggle to get XP in instances (only be able to do them when almost too high) and earn all XP at half the rate a single character would. That means that you can burn through quests easily, after all there is two of you and that makes the quests easier, but because you get much less XP off the mobs you need to kill you run out of quests in the zone long before you're ready to do the next zone.

At 20 my hunters had done most of the quests in Westfall, Bloodmyst and some in Darkshore, thats just the 10-20 section, they'd already done all the quests in Dun Morogh (1-10) and some of the quests in Azuremyst (1-10). One thing you get to do is see more of the game. I did my first DM run at 26-27 and got a small amount of XP off some of the higher mobs, but mostly the XP was crap.

And that is with most of the collection quests done.