If you like casting, try two mages. Like a wizard and a pet summoner (necro if evil, summoner or ... conjurer? if good - sorry I'm blanking on the exact name of the good summoner).
I've yet to try it, but I wonder if two wizards can "ping pong"? Hey, that'd be an old eq tactic they managed not to squash, maybe. Ping-ponging = one wiz pulls w/ massive nuke. As critter runs at him, other wiz, who is the "right" distance away, zaps critter w/ his massive nuke. Critter is so mad that he runs after him. But he gets zapped by the first guy - and he ping pongs back and forth either never hitting either caster or only doing slight damage, til dead.
Two wizzes would = fast damage. Hmmm. You could also AE (area cast) on swarms of greens possibly.
The previous poster, howver, makes really good sense. Healer and tank/dps type makes a good, solid combo. A berserker or monk/brawler/bruiser might work best cuz they do more damage than guardians/pals/sks. And they have more HPs and capacity to take damage than a scout. They can taunt, too. But a guardian, sk or pal and a healer would work, too - the healer might need to nuke a tad more to kill faster - and wouldn't have to heal so much because the tank had more hps and ac, etc.
Note, use player made food and drink of your tier (lvl 10 to 19 is tier 2, so drink antonican coffee, whisky, etc - eat cichlid steaks, vulrich steaks, carrots, candied oranges, etc.) - u will have less downtime cuz this food = faster health regen and drink = faster power regen (faster than store bought or summoned food). I mention this because u want fast, efficient killing - less downtime is the key, too.
With this in mind, two scouts might possibly make an okay duo. As long as you kill stuff fast enough, you can use player made food to regen health between fights....
And a mage/tank combo is a possibility now, again, if you can kill stuff fast enough. Or a pal/mage...? That'd be weird - the pal can ward himself... I wonder...? But note, two scouts or mage/tank is very experimental - maybe join test server and try it out there? Heck, I might just do that.
Speaking of efficiency, I'm thinking of playing up a wiz and trying to form AoE groups. Six wizzes w/ area nukes ... or six wizzes on ^^ mobs w/ normal dd nukes... it might not work in all situations. But 5 wizzes and one healer might....
Berserker/Shaman might be your safest bet. Zerker is decent dps and some tanking ability. Shaman can heal, ward, slow the mob, nuke, DoT - and cure nasties - and SoW - and water breath. Overall for this game a well rounded combo. Any healer would work - they all heal well - and they all have nukes.