Two of any spec will destroy most content while you're questing. Spec will then be subjective actually.

I had a 2 warlock team before. Both Affliction. it went something like this:
1 Void, 1 Imp, two affliction warlocks. this was used up to level 20s. then i got the succy for more dps. so 1 void, 1 succy, two affliction warlocks.

basic battle goes as follows: send up void and succy/imp, immolate right before they reach the mob, corruption, drain life til dead. they die too quickly for you to even use the other affliction spells.

at level 45, my whole fighting style changed as now i have instant howl and dark pact. 2 imps on phaseshift, 2 locks. the tanking lock should always have more stamina and spellpower than the non-tanking lock. yes, locks can tank.

i matched pretty much every spell with both warlocks. only there were two things different. they take turns with fears, howls, and deathcoils. they also take turns on who does drain soul, as only one warlock will get the +15% mana return bonus. both will get a soul shard, however.

at 45 and up, my battle goes as follows for both warlocks. SL> CoA > Corruption > move to next mob, repeat until you get 4-6 mobs. instant howl > Dark pact > drain life
note, when you drain life, your slave lock will drop follow. so if you need him to follow again, make sure you have a macro to follow.

you can go on indefinitely with this setup. with the first 4 mobs feared by HoT, you can move on to 4 new mobs and HoT them with the slave lock (after having them DoTted up, of course). any problem mobs will require a fear, and not a HoT. Use Deathcoil for fun.

macro for fears are easy. if i want the main to fear, i press the hotkey. if i want the slave to fear, i use an ALT modifier.

main: /cast [nomod] Howl of Terror
slave: /cast [mod:alt] Howl of Terror
easy enough? make this for fear, deathcoil and drain soul.

drain life requires nochanneling.
slave: /cast [target=focustarget, harm, nochanneling: Drain Life] [nochanneling: Drain Life] Drain Life
main: /cast [nochanneling: Drain Life] Drain Life


At 60, i open with Haunt.
UA? doesnt matter. get it if you want.

Hope this helps.