No clue but the court that I learned how to play ball on had a double rim. Did wonders for my jump shot and mid range game. So if I could give any advice from my own experience is practice on a double rim it can only improve your J.
not all double rims are same either. if they simple take 2 rims and put them on top of each other, it's not that huge of a deal. there's this one park i go to has 1 double rim that's and inch tall.... ****ing hate it. worst thing about that court is that it's tilted. messes up ur perception 2 ways.
The double rims that most courts have these days do a great job of sustaining their original relationship to the backboard. They don't bend, and that's a dream come true for public outdoor courts. Before these buckets came around, my area was just littered with rims permanently bent to 9'7'' because some little feller climbed the net and hung up there for a half hour.
And like 70% of the posters in this thread, I truly and honestly enjoy double rims because it perfects ones' shot. There's a certain gratification that comes with watching the ball fall into the hoop, even if it bounced around the rim a couple times first. It seems to signal your shot was good enough, even if it really wasn't that perfect of a shot. Indoor rims seem to lower one's own shooting expectations. Outdoor buckets raise them.