For USPSA competition, the North Mountain holder is about the best rig available.
North Mountain Moon Clip Holder - 4 Post - MCH4P-625.627
It holds 8 clips securely, it's very fast, and you don't have to search around your belt to find a full clip. This particular model is a bit overmuch for concealed carry, although they do make it in 4 and 2 clip versions.
The Moonclips.com holder or the similar California Competition Works holder are OK for concealed carry, but they are not as secure as a snapped holder. Matt Del Fatti makes one of the best examples of this, and he's a member here, but there is a long delivery time.
Del Fatti Leather
BladeTech makes Kydex double moonclip holders. I haven't used one, but their holsters are very well made.
Moon Clip Pouches :: Double Mag Pouch :: Pouches :: Blade-Tech Industries
Buck