I've spent some time but I haven't really found any clues to my question: Should I Multibox? (probably not more than dual)

Here's what I am thinking of doing: Leveling 2 characters simultaneously from 80-85. My main is a Disc/Holy Priest but I'm pretty sure I'll level as Shadow. I have a second Priest that is almost identical. It is a long story how I ended up with 2 Priests but it has worked out well to have a second set of raid IDs. I also have an 80 Warrior that was my main back in BC. I'm really only practiced playing Protection. And I have a 80 DK that I could run.

My first preference would be to use the second Priest. However, if using either the Warrior or DK would speed up the leveling then that would be a big consideration. I would imagine running 2 identical Priests would be the easiest combo in terms of ease of multiboxing as well as speed.

I have the computer hardware to easily handle dual boxing. I have never tried it however. I run multiple accounts sometimes by alt tabbing (I imagine that made some people cringe) but that is obviously not the same.

So what it boils down to is:

1. Can you level 2 toons at once by multiboxing faster than you could level 1 toon running solo?
2. If not, how much extra time would it take? 10% longer? 50% longer?
3. With Cataclysm approaching in about 6 weeks, do I have enough time to learn to dual box efficiently enough to be able to meet the answers to 1 and 2? Again I am assuming running 2 identical Priests would have a big impact on this.

Many thanks to those that offer opinions or guidance!