Need advice: 2-boxed Shadow Priests

I have two shadow priests I'm leveling up together (currently level 22). The combination is wicked nasty, but I'm looking for advice from more experienced botters concerning specs.

Most grinding builds for shadow priests are built around mana efficiency and wands, using spirit tap to restore mana very quickly after a fight. Unfortunately, this requires that a character gets the killing blow, effectively cutting its efficiency in half when using two boxes.

Likewise, I kill so fast that most things die rather quickly, making wand spec fairly useless, and improved shadow word: pain even moreso, as nothing lasts long enough to get more than 3-4 ticks of SW:P normally.

Anyone have some thoughts on a 2-box friendly setup for dual shadow priests? I'm kinda stumped.

Just use SW:P, then wand.

2x SWP and wands will kill most stuff really quickly (until higher levels). Once mobs stop dieing quickly add 2x mindblast as the opener(s). The key is to keep buying the best wand you can get and having 5/5 wands. Using this strat I rarely ever go oom on either toon.

Riault

I leveled 3 Shadow Priests to 60, it was quite effective but wasn't my tastes. As for leveling 3 improved SW:P's was enough to drop any enemy my level, so my tactic was to just dot, and go to the next target. After receiving the Vampiric Embrace talent I would slap that on every enemy I could when it wasn't on cooldown making it so you didn't need to heal :)

With 2, SW:P by itself might not be enough unless you invest into some damage gear, I pretty much played with really bad gear to 60.

With two, you can easily do two same-level (and maybe even above) targets at the same time. Open up with MindBlast x2 on target A, then a pair of SW:P on target A, pull in target B with a pair of SW:P as well, then mind flay target B as you cycle fears. Both should die at around the same time.

It's not the most effective duo, the dps is nowhere near what a hunter or mage or lock duo gets, but for pvp there is in my opinion no better duo. Five twinked rogues vs my untwinked level 29 clothies in AB was no problem.