A few tips please from the more knowledgable

[DarkFish]

Right, I'm an absolute beginner at this. So far I've bought a second account, and am planning to dual-box it with my main. I have purchased a license for Keyclone after reading this forum somewhat, and am fairly confident I can set it up well enough to get going. But what I would like to know are some of the folowing:
Class combinations to avoid and to use (in my case I plan to play two of the same class, but if I ever expand my team it still would be nice to know anything for teams up to 5)
Professions (What works well? Are gathering professions fiddly with the mouse-clicking?) Talents / Spells to avoid (Things that are just a waste of time or difficult / impossible to use)In the case of these I am not looking for hugely detailed answers, just short clear responses. For example you might say "AoE targetting is a nightmare" (or so I've gathered?).

Also suggest from experience a nice two-team combo, keeping in mind I ideally would play the same class because I am new to this and presume it's far easier to manage! ;)

Thanks in advance, looking forward to a new experience!

[Gallo]

2 boxing allows for just about any combination. The only thing that I would avoid is 2 melee characters. If you choose a melee character, you'll need to use that as the "main" while the ranged guy stays behind you. Beyond that, you can use any 2 classes you want. To make things more interesting, you could use 2 classes that you've never used before.

Good combos include:
-Warrior / Priest (dmg/tank with a healer/damage dealer/rezzer)
-Shadow Priest / Warlock (talents work well together, soulstone on the rezzer)
-Mage / Mage (2 POM Pyros to the face, great AOE)

But 2 characters that maybe you haven't played before, you could do stuff like:
-Rogue / Hunter
-Paladin / Mage
or anything.

Professions:
Have 1 character do mining, the other herbing. Then, choose the crafting profession that applies for that class (LW, Tailor, or BS).

Before I started 5 boxing, I 2boxed a shadow priest and fury warrior. It was pretty good fun.

[-silencer-]

What Gallo said - Paladin/Mage - has been my favorite 2-boxing pair. An AoE/prot spec paladin combined with a flamestrike or blizzard mage is a ton of fun, especially once you hit lvl 30 and the paladin is capable of tanking quite a few mobs and hold aggro through flamestrikes/blizzards. My combo with those two was Mine/Smith & Tail/Enchant.

[Boylston]

Here's my 2 silvers worth:

Class Selection: The above posters have made good suggestions, I'll add a few thoughts. Definitely only do 1-melee! Two of the same class will be the absolute easiest to set up and maintain macro-wise. Two locks is probably the easiest due to the fact that a lot of your DPS doesn't require that you face the target (affliction's DoT spells can be cast behind your character, for example). One final note, if you currently have an account with a high-level character, you might want to keep in mind that you may someday want to get another account and transfer a character to that one. My own personal example follows:

I had 2 accounts previously, with #1 containing a L70 Shaman and a L70 Pally. I purchased 2 more, and have been leveling 3 more shamans on accts #2,3,4 that are little orc-clones of my L70-- I can eventually run 4 L70 Shamans when they finally finish. Right now, I can also run the L70 Pally with the three little guys to help them with quests or what not. Someday, I may decide to get a 5th account and move the pally to it so that I could run 1xPally, 4xShammies and do my own high level instances solo.

So just keep in mind that if you already have a high level character, they will eventually be able to play with whatever dual-box team you make, assuming the account info has your (same) name on all of them.

A healer/tank combo is really nice as well. You'll be able to run instances nearly at will if you have the two hardest slots to fill already locked up. There's rarely a shortage of DPSers willing to run instances. It's also a great way to level/quest by yourself.

Professions: I'd put enchanting on one of your characters for sure, just to be able to DE loot you get. The rest are up to you. With dual-boxing, it's not so daunting to level tradeskills up. My shammy team has two members with no tradeskills, though I will eventually plunk down the cash needed so they can each DE their own outlands quest rewards (this should generally pay itself back). I'm leaving some open slots for the expansion profession as well.

There's no shame in rolling 2 of the same class and trying out the macros/setup that is required for a few days. Roll a couple of mages and get a feel for how /follow, /focus and focus-assisting, and combat work. You may also want to consider rolling 2 druids, since with talent/gear choices, you can effectively play a Tank/Healer, Tank/Caster, or Caster/Caster combo easily multi-boxing. (Druids are nice healers as well since they have a lot of instant-cast HoTs that can be done on-the-run while your Feral kitty main moves from target to target.)