I know this question comes up a lot on the forums, since I've raised the question myself. I found this site showing the gear configuration for doubles, and it includes the theory behind why it is recommended to do it this way.
Here is the link. http://dir-diver.com/en/equipment/reg_config_doubles.html

This is a very informative site. The main page is http://dir-diver.com

I'm far from being a DIR diver, but I do agree with some of what this site has to say. At the very least, its another perspective, and its interesting reading. there's some interesting DIY (do it yourself) instructions that I plan on trying. I enjoy making (or trying to make) some of my own gear. I plan on trying the stagebottle rigging (as soon as I actually buy a stage bottle).