I have now managed to get my raf dual-boxed toons up to 20 and have even managed to figure out a few neat things. Since someone asked, I shall share them with you.
1. A druid/priest pair is ridiculous fun. The druid bear-tanks and kitty-dpses, and the priest dpses and heals. It's a nice combo. (and yes, I have forsaken the warrior in favor of the druid. What can I say, I had way too much fun with swift flight form on the ptr.)
2. Hotkeynet is a really powerful program that is capable of many awesome things, and thankfully its web page has scripts for most of them. I had to do some tinkering (my keyboard lacks a scroll lock key and I use different movement keys then the default), but it wasn't too hard. I even got picture-in-picture to work (which makes life so much easier). Plus, it's a free program. Yay free.
3. Macros are key. Seriously. They don't have to be anything complicated, and in fact most of mine are things like...
or /assist Monera
/cast Shadow Word: Pain
Nice and simple, but it means that I can just hit a button for the priest to heal the druid or to target whatever the druid's targeting and cast Shadow Word: Pain at it.
4. Professions. Well, I've already outleveled my herbalism on the druid, and the less said about the priest's mining, the better. It might actually be easier to just go back once you hit 60. But if you're going to do professions, do ones that offer your toons armor upgrades. The priest is also a tailor, and most of her gear is stuff she made herself. Fewer quests and kills means your gear is not going to be as good, so making stuff yourself saves gold.
So far I have thoroughly enjoyed multiboxing, but there are a few caveats....
One, having someone try to talk to you while you're doing it (especially if you're using hotkeynet or another keycloning program) is a bit of a nightmare.
Two, without macros and addons (I suggest Jamba), you will be switching back and forth a lot.
Three, you will generally not learn as much about each class as if you were playing them individually.
Four, it gets expensive. My toons are certainly not self-supported, and if you're doing this on a new realm, roll a DK first for the cash.
Five, it's far easier to have all the gathering professions on one toon. Less switching back and forth.
And Six, you need to have a decent PC to do it. Mine is not a top of the line, but it is a dual core with 2 gigs of RAM, and an okay video card.