I've been multiboxing a Prot pally, Holy Priest, and Arcane Mage together for some time, so I'm quite used to a tank in the mix.

For my $0.02 worth, I'll tell you how I manage... the tank is my master. I just couldn't conceive of running into battle any other way. All my other toons have /assist TANK_NAME /cast SOME SPELL macros set up for their "opener" dps. Every time I change targets on my tank, I can just whack that to focus fire on the new mob. If I then want to switch tank to something else, I just use other spells than the one with the /assist.

My setup for entering combat is to stop just outside of aggro range, then hit a quick "backstep" on my slaves to get them to stick where they are, then range or face pull with the pally... range if Im not worried about aoe/cleave, face if I am.

Nice thing with a pally is that if my slaves start to pull aggro on something, I hit consecrate and my "follow me" button, and suddenly my squishies are right next to me in my consecrate, and the mobs generally tend to forget about them and go for my pally.

I've also had luck when I was first leveling my priest using a rogue/priest combo. There, I had the rogue stealth, the priest would pull with shadow word pain, and when the mob came running, the rogue would cehap shot and I'd dps like hell, hoping to kill them before they murdered either. It was a lot dicier than runnin in with my pally, but it worked.

Either way, for me, there's no way I'd have a tank/mele on any by the master. With everyone else following, they're automatically "on point" and I have the least amount of repositioning to do when combat begins.

Hope that helps.