I have been thinking about this for two months now. I'm maining a Warlock and Alting a Priest. I KNEW enchanting and tailoring was going to be in there somewhere, but I couldn't quite decide on another two professions and how I was going to spread the professions across the characters. Kargoz suggested putting the alt on tailoring and enchanting, while throwing the main on skinning, or something close. This was in order to get your wands in a row early in game, but I tried this and I hated having to mail so many items back and forth. It made toons reliant on the other and wasted copper.
I eventually decided to have my main Lock go with herbalism and enchanting, with my priest doing the tailoring and alchemy. This seemed the most efficient. The only items being mailed back and forth to the alt would be herbs and returning potions, with the cloth going on a one way trip. Looking at the pre-raid BiS craftable items from enchanting, it seemed priest had more gear than what I wanted for warlock, and with those BoP items, i'd rather have them on my priest. My lock gets great items from dungeons and quests. Of all the items my lock needed beyond wands and cloth sets were potions.
So, my main will have access to all the high end herbs and cloth mobs (and can handle handle the mobs much easier), and my alt will give my lock the potions needed for raiding. It was one of the very last details that I needed to work out.
I'll probably make a human warrior eventually, but professions for a third toon seems like a waste of resources.
