I experimented with my supertuck and minituck, inserting the minituck on my weakside hip and holstering my SR9c in the supertuck on the strong side. My pants weren't overly tight and the belt held and secured both IWB holsters well without any discomfort or weirdness. The minituck was there to kind of simulate what a IWB mag carrier would feel like.
I have a dual mag carrier made of Kydex. I don't like it for concealed carry because it locates the magazines too far away from my body for proper concealment. Plus, having two magazines that stick out that far tend to get in the way and poke & scratch at my arm. I can only imagine the kind of havoc they would wreak on my shirts.
But Crossbreed seems to have hit it on the head with their OWB design. The mags seem like they would be pulled in tight against the belt and would ride close to the body without scratching (due to the leather that goes up and over the magazine.
However, the real big difference appears to be where the magazines would ride with each carrier. The OWB seems to make the magazines ride high so as to make concealment while OWB possible. The IWB carrier, on the other hand, appears to make the magazine(s) ride lower since they are covered by the pants. It seems to me that the only part of the magazine visible would be the part that is not covered by the kydex. With the OWB carrier, a good 2/3 of the magazine(s) would ride above the belt line.
While having the magazine(s) ride high is good for tactical use, it doesn't lend itself to concealed carry as well as the lower riding IWB setup. I know from experience that the high riding magazines can jab into the kidney area or ribs, depending on your body type. They don't always lend themselves to comfort in a vehicle or in a situation where you might be sitting down for prolonged periods (say, at a movie theater).
On the other hand, a lower riding magazine may not be as easily pulled from the carrier, which could add critical seconds to a reload, and could potentially cost you your life. Then again, when I think about it, the whole act of concealed carry tends to slow you down a bit anyway, and 99% of the time, comfort and concealment win over tactical ergonomics and ultimate speed. In reality, with enough training, one can become fast with concealed weapons and carriers, so speed might be a moot point.
The other question would be this: go single or dual? There isn't much of a cost difference between the two, so money really shouldn't a factor in the decision. The decision comes down to whether or not I really need two extra magazines for my concealed carry gun. Do I need to carry all that extra ammunition with me just to go to the grocery store or the mall? Maybe... maybe not. Perhaps one could order the dual and just use one of the mag holders available? Good question.
I'm still torn. Go IWB or OWB? Go dual or go single? I used to carry dual mags weak side with my Beretta 92 FS riding IWB strong side - no joke. It was 46 rounds of ammunition carried on my person when I went out like that. That's a lot of lead. With a SR9c, 10rd mag in the gun, and two 17rd backups, there are 44 rounds available. Decisions decisions.