Demon Locks are fun, too - the Felguard can handle a lot, he's a DPS machine. I tank BC dungeons solo with my 80 Lock.

I started out with 2. It's fun, and questing is VERY efficient, regardless of class.

To my mind, your main can be whatever you want, but a healer class as second is VERY handy to have, if possible. I tried 2 pallies, 2 mages, and 2 shaman, and while they're great at the lower levels, they can run into problems later on - and the synergy you can get from 2 different classes is more fun and efficiency.

Some classes also have very handy utility, like resurrection, buffs, and with mages, food, water, and portals. A mage is a good buddy for all casting classes, for the water and Intel buff. Priests are also good buddies for their buff. Druids have Mark of The Wild. Pallys are obviously very handy with their buffs and heals and tanking. Shaman are great for their totems.

Some classes are weak in the beginning - a priest is a little more of a challenge until you start getting the good Shadow talents and can melt face. Other classes like Pallys are OP from the start. Rogues are a challenge at first if you've never played one, but the can be a lot of fun in a pair - stealth up, pickpocket, sap or garrote, nuke. Rinse and repeat. Lots of fun! A rogue or warrior with heals is a killing machine, very little downtime and very fast questing.

I would say, level one of each to 20, then just start mixing them up. The key here is, a team is what you decide it is - if you want to switch the mage out for a shaman, or switch the priest out for a Boomkin, go for it. Find the combo you like, and level them first. All the while you're still getting RAF, anyway. If you want one of each on both accounts, you'll have healers available no matter what. Run into a quest a certain team can't handle? Swap in a toon that can handle it, finish the quest, bring back the other player, swap out the other.

Most of all? Have fun!

Can you make a DK on your first account? If so, level one to 70 or 80, and you can take your guys on the other account through most of the vanilla dungeons - and he can run dailies to fund both of your accounts.