Many of the cleric DPS souls are late bloomers. Shaman will be sluggish until about level 25. By then you'll have jolt, brutalize, and lust for blood. Frozen embrace will also be a strong boost to damage. I'm not as familiar with melee clerics, but you might have better luck focusing on Druid early on rather than shaman, making sure to use both bombard and eruption of life.
If you just want to dps and don't really care how to do it, I found leveling as an inquisitor to be quite good. At level 26 it's starts to get really good.
Also, depending on how willing you and your friend are to changing strategies, you might want to mix up what you're doing. In a duo, having 1 full dps and 1 full healer is fairly bad for killing monsters individually. Many dps can solo mobs almost nonstop by themselves, and with additional dps, the healing from a healer really goes to waste. All you really need is a single light heal to top yourself off between fights and you're good, and you get get than from 0 point in any of the healing souls.
To be more effective at leveling you have two options. If your friend is willing to dps (while grouping with you, he can always change back to a healer for instances, pvp, or any other time), then you should both definitely go dps if you're planning on killing mobs one at a time. You don't need the healing, and having two full damage dealers will make it go much faster.
If your friend doesn't like playing dps and only wants to heal, then you have one alternative to be more efficient. That option is to aoe pull and aoe dps. By killing groups of mobs with you doing aoe instead of single target dpsing them down you can actually make use of his heals (because you will be taking a lot fo damage) and you will be killing monsters much more quickly. Inquisitors can do this fairly well with soul drain and circle of oblivion (requires level 31) to up to 7 mobs. Cabalists are all about the aoe, but seem to target around 3 mobs most often (I'm not as familiar with them). Druids and Shamans can hit up to 3 enemies in the teens, and shamans get another aoe hit that damages 6 on 10 second cd when they turn 21. Shaman should do fairly decent aoe if you wanted to stick with that.