I'd also definitely go 2 of the same class - trying to remember what you have on each toon's hotbar in each position can be horrid if you're not really anal!

I'm fairly new to multiboxing - my plan was to level a healer to 60 in the quickest time possible so my guild could get started with guild only runs....

4 different toons later I'm still on my 30 days from the WoW box - contemplating whether to get a time card to get a few more toons - RAF leveling is SO easy - and remember - you only need to do 1.5 toons to 60 to get 2 toons - the RAF free levels will get you the rest of the way.

Enjoy multiboxing, and be prepared for everyone else on your server to be amazed that you have so many high lv toons