Having a hard time decided on two classes

I dual boxed once before.

Basically - I just ran two instances of WoW on the same machine and clicked on the screen I needed. I did a Mage/Mage and just went back and forth hitting "2" to cast fireballs.

I'm considering trying it again (with the sped up leveling from 20-60). But I'm having a hard time figuring out what class to pick.

I'd like to attempt to solo some instances later on (not the really hard ones - but ones that two characters could probably do), and I mostly solo.

I really like mages - but having two kind of seems like a waste. I'm thinking maybe a mage / priest. But it's just deciding on two classes that's tricky for me. I'm not sure which two would go well together, be easy to manage, and would be able to do some fun stuff later on at 60 and 70.

People use the word "waste" differently from me. When I think of "waste" I think of "not being used". The only way another mage would be a waste if it was going to do nothing but stand there and look pretty. 2x of a class is a good way to make things simple for yourself. I would recommend it for the ease of use, and having same spells and cooldowns.

I wouldn't see 2 of any class as a waste. Matter of fact 5 of any class wouldn't be a waste. Just think that each class change have a number of different specs from any of the three trees. You can have a Frost Mage / Fire Mage or any combination there of. Maybe you spec one once you get to 70 as a PvP character (Frostie) and the other as a Raid character (Arc/Fire maybe). Think of all the money you will save from not having to respec over and over.

Gives you a good chance to see multiple aspects of a class, and eases the configuration.

If you want to do some 2-box instances later, then try to get Tank-Healer-DPS abilities into 2 classes.

Let's start with a tank.

Warrior. Tank/DPS
Paladin. Tank/Healer or Tank/DPS (more Prot Spec, and rest in Ret tree).
Druid. Tank/Healer or Tank/DPS
I suppose the other tank you can use is pet, Hunter or Warlock.

Healer.
Priest - Holy/Shadow does not seem to mix well. Holy / Smite is viable.
Druid - Healer / DPS, or Healer / Tank
Shaman - Resto / DPS
Paladin - Holy with some DPS Sanctity Aura and Holy Shock build, or Holy with some Prot.

Sanctume sums it up well. My first Dual Boxing was with a Holy Smite priest and a Prot Pally. I would say that I highly recommend it :)