In reply to jim:
Yeah, you just use two similar sized krabs back to back clipping the rope/harness with both.
I've only tried a couple of the modern grooved devices: One I got for my girlfriend (Red, probably BD or maybe DMM - Sorry, useless info!) when she started which is very nice and progressive. The other some ghastly stainless steel junk (possibly also DMM) belonging to a friend who presumably found it in a xmas cracker.
Personally I use an old (unsprung) Sticht plate for half ropes and where weight matters, otherwise I us use a GriGri. In both cases it's the powerful initial bite that I like, neither lower brilliantly with out a bit of practice but that's secondary.
jk