Theory of multitabling...

[Pokerfarian]

For $ multitabling is just the way to go. There's just no way a good, serious player could not make more than half as much on each of 2 tables, so 2>1. It probably doesn't take too much getting used to make more 4-tabling. Then it becomes personal preference, you can go ultrahigh.
However if you're trying to learn a new game/limits/improve then single tabling is actually good. You can actually think about why you're doing stuff.
Obviously it also depends on what games you're playing. x-tabling full ring is not the same as x-tabling HU.