Actually thinking about it, I think maybe my best bet is to dual box 2 tanks at a time. This would save me double the time by using 2 toons at once and it would not be as much effort as running a full team.
This is your best bet. If you want to fill out your stable, get an RAF setup for the two characters and then just dual-box them together through quests. If you go the DK+Pally route, you also have the flexibility of speccing the DK as Tank, the Pally as Holy, and just using the dungeon finder tool to grab 3 PUG DPSers if you want.

RAF+Questing+Dual Boxing means that this little pair will level very, very fast, and you can even do collection quests without a huge headache.

Making a 5-character multiboxing team for the purposes of "leveling fast" is not a good idea.

You might also consider some powerleveling with one of your existing tanks on account 1 while account 2+3 (RAFed together) soak up the XP... Then quest from 58 or so onward.