Personally, I roll on my ONE most appropriate toon on a given boss/item, unless explicitly worked out beforehand.

If a cloth item drops, I'd either roll on my mage or priest if it was an upgrade for one of them, but never both.

Truthfully, I don't usualy have that issue as I solo-play my priest and she's full 213+ epics, and my mage is barely heroic geared... I'm not likely to be multiboxing anywhere my priest could actually USE an upgrade (Maly 25, Naxx25(KaelThas has my only upgrades), and Ulduar).

More likely, I'm "solo" multiboxing classic and BC content (though I bring my team along on "old skool raid night"), but none of my toons need anything pre-WOTLK, so it's a non-issue.