Trying to complete my 9 classes

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out of Warrior, Shammy, Pally, Mage, Rogue, Hunter what 2 pairs are best to box with and with 2 toons should I just grant levels to?

For grinding dual-box (something I did for a year before 5-boxing), I always used unconventional hybrid specs to try to get the best use out of the main. Here's the characters to make:
Acct1(main)::Acct2(follower)
Warrior(arms/fury)::Shaman(enhance/resto)
Pally(AoE prot/ret)::Mage(AoE:AE, FN, CoC, Bliz)
Rogue(com/sub)::Hunter(BM)

The warrior/shaman combo:
I'd spec the shaman either 17/22/22 (for improved fire totems, improved weapon totems, heals) or 0/22/39 (improved weapon totems, heals).
I'd spec the warrior 40/21 (improved mortal strike, sweeping strikes).
The idea is for the warrior to use a massive 2hd for high damage with some multi-mob skills, then proc off the windfury totem.

The pally/mage combo:
I'd spec the paladin 0/41/20 (BoS, HS, Reckoning, avenger's shield, improved ret aura, crusade)
I'd spec the mage 13/0/48 (arcane impact, frost talents)
The idea is for the paladin to use an AoE tank build for damage/threat generation and the mage using frost nova, shatter, cone of cold, blizzard to burn 'em all down.

The rogue/hunter combo:
I'd spec the hunter 48/13/0 (nearly all beast talents, improved hunter's mark, go for the throat, aimed shot)
Now, there are 4 rogue builds I like to use:
- 15/25/21 (murder, lethality, imp SnD, sword spec, and hemo)
- 15/41/5 (murder, lethality, puncturing wounds, combat potency, surprise attacks, aggression, blade flurry, dagger spec, imp. slice n dice, opportunity)
- 15/0/46 (PvP-oriented dagger spec.. murder, lethality, puncturing wounds, hemo, imp. ambush, shadowstep..)
- 41/20/0 (mutilate, improved poisons, master poisoner, seal fate, imp. KS, dagger specialization)
The idea is to use the hunter's pet to tank, while you have your rogue as the main to DPS one at a time. An AoE-threat pet like a carrion bird or owl works best, since it won't be as likely to lose aggro on adds when you bandage. The hemo build works very well with the combined physical attacks of hunter + pet, doing a lot of extra damage while the rogue has steady reduction of threat through feint. My favorite is the 2nd hybrid build for massive amounts of backstabs + combat potency. You *need* a 1.30 speed off-hand (Blade of Unquenched Thirst, Warp Splinter's Thorn, Searing Sunblade, Edge of Oppression), and as close to 363 hit rating as possible - the main goal of the off-hand is hits per minute to proc combat potency for more backstabs. Don't neglect agility, AP, and crit % for hit rating too much though. Add in adrenaline rush + blade flurry and you're dishing out a lot of multi-mob damage. The third spec also works very well and is easily the best for PvP of the group. The fourth spec (mutilate) does very well until you meet poison immune mobs.. so I haven't used it as much.

Now, you'll only be able to grant levels from the shaman, mage, and hunter. Since the RAF xp bonus only works if two characters are leveling together and are the same level, it's a tough situation. The best overall thing would probably be to level the warrior/shaman team to 60, then the paladin/mage team to 60. Now, level a rogue and hunter solo to 30 on your main account, and grant 30 levels each to get them to 60. If you want to actually play the rogue/hunter together, you can level that pair to 60 instead of the others, or transfer one to the 2nd account after they're granted up to 60.
So, the plan would be like this:
Warrior/shaman to 60.
Paladin/mage to 60.
Rogue/another class(RAF bonus for rogue while leveling) to 30.
Hunter/another class(RAF bonus for hunter while leveling) to 30.
Gift rogue to 60.
Gift hunter to 60.

Account 1 will have:
60 Warrior
60 Paladin
60 Rogue
60 Hunter
Account 2 will have:
60 Shaman
60 Mage
30 random (druid?)
30 random (warlock?)

If I were to have duplicate classes, I'd pick among a paladin, druid, shaman, and warrior. That way, they could be dedicated to a specific spec.. like a paladin AoE booster for new alts.