Knoll makes a very, very good one:
Knoll : KnollExtra : Sapper™ Monitor Arm Collection I'm not sure if it supports two 24"s though.
Humanscale also makes an almost-as-good one:
http://www.humanscale.com/products/p...p=m8monitorarm I'm pretty sure this one supports two 24s.
Both of these have the option of clamp mount btw.