I've just started 2 boxing with two mages and here's the only tips I can provide thus far. Really, you can dual-box any two classes and make it work as long as you're having fun. Generally it's good to have someone who can heal and rez which the shammy is taking care of. So you may consider having a dps'er like a mage or lock standing in the back spamming nukes. Priest wouldn't be too bad either, still decent dps but now you have dual heals + rezzes which is nice. Just kind of think about what you want to be doing at 70 and build off of that. I plan on aoe farming / daily grinding / instance running lowbies so I chose 2 mages. That way when I make new toons on separate accounts I will always have a 70 mage who can run them through an instance. If I ever get around to buying a third account I'll probably move a holy pally over as well so I can have a mage/healer combo at all times. If you're going for more pvp based then 2 shammies work well, or maybe a druid. Keep in mind that when someone runs up and starts hitting you in the face, you may have problems where your slave is facing the wrong direction and you need to move him around a bit, so I personally would pick something durable that can heal so you both can outlast through those sticky situations.

Ultimately, I say pick a class you love to play and throw that in to give it a try. If it turns out you don't like it or find it a weak pairing, you can always roll a new class. If you've got a shammy out of party carebearing a new toon, lvling it up won't be much of a problem.