Talking with the folks from Galco at SHOT Show in January, and one of the company's new offerings for 2015 is the KingTuk 2 holster.
In response to numerous customer requests, this IWB holster contains a hook-and-loop section that can accommodate a spare magazine holder. Building on the popular KingTuk line, where the holster can be covered by a tucked-in shirttail (as opposed to traditional IWB holsters where the cover garment is loose), the KingTuk 2 allows a spare magazine to be part of the standard gear rather than a separate piece.
Having all gear in a "tuckable" format makes for excellent concealment, although it does make presentation slightly slower. If your concealed carry needs dictate deep cover, this is certainly an excellent option, as all that shows are two belt clips. For even greater concealment, a cover garment over the tucked-in holster means the odds of inadvertent exposure are very low.