Re: Rear coil over shocks q ?
I don't have any practical experience with these, however I have some theory.
If you are going for more than drag racing, the doubles are probably a waste. However if you are planning on autocrossing, or road racing then they might work out well for you.
With a double adjustable you get to adjust both bound (compression), and Rebound ("bounce back") speeds. For the track you can set the bound to full soft, and the reboudn to full stiff (or some combination along those lines), and the weight will transfer back quickly, but not rebound to fast, and "jounce" the truck. Then for autocross/spirited street driving you can set both to near the same, and have a decent handling truck, with perhaps the bound set a bit higher than the rebound.
If you have the cash, and think you need the extra adjustability than go ahead and get em. If not, savea few bucks, and have rebound adjustment only, and call it good. I don't think you'll be unhappy with either.