I am probably way out in left field here, but I don't think this is possible. If tabs are not nested, they are basically just block sections of the profile (<div></div> statements in html). You can only adjust which one appears first in the tab administator.
This brings up a good point though -- does anyone have a simple explanation of how to adjust the basic layout of the profile?
I've noticed on my site that kids can cut and paste "myspace" layouts into their profiles; some look good, and some not so good. I've also noticed that when you get into the tab layouts in administrator, the layouts are numbered in the far right column. Given a ton of time, I could probably write a treatise on how they work, but I don't have the time right now to play around with it.
There is nothing in the manual about how to do this, but knowing how smart the guys are who built this thing, I have to believe it is not random...