For CCW, I’ve always carried my extra mags 1 of 2 ways: in the pocket, or in a Galco double mag carrier attached to my belt, weak side. Obviously the mag carrier is not that concealable, and the pocket carry has its drawbacks.
I’ve noticed 2 other common ways to make mags a bit more concealed: horizontal carry, and IWB.
Do either of these alternate methods work, or will they have enough drawbacks that I’d be better off to stick with standard mag carrier?
What are you CCW’ing? How many mags?
M&P 9, 2 mags in the carrier.
I’ve played around with a variety of mag carriers and was for the most part somewhat disappointed with the concealability of them, as a result I moved to the pocket carry until i got an RCS single mag carrier. This mag carrier is held much closer to the body than previous others I have tried. I would definitely give them a try.
I’m not going to go into what is an appropriate amount of rounds for carry. I will say that there are trade offs and everyone has their own ideas. Do I personally see a need to CCW (with my own emphasis on “conceal”) a full size pistol with a 52 round count? No, but that is me. You may live in a higher threat area than I do; I don’t know. Lately I have been on a minimalist kick when off duty, that is usually a 5 shot .357 snubby with a 6 round speed strip in a pocket. I can carry all day in shorts and a t-shirt and nothing can be seen. At work I generally carry a P2000SK .40S&W in an IWB with a single extra mag on my ankle, again, jeans and a t-shirt and no one knows I am packing. Is what you want to carry truly concealable? With how much effort? Is your M&P holster IWB? Adding a double mag IWB in addition may increase the size of the pants you’ll be wearing a waist size or two. Comfort is a consideration when CCW’ing all day, as is concealment. You may want to reconsider what you want to accomplish, your needs, and your equipment.
That’s the Raven right? My son has their holster and single mag carrier and I’d say that mag carrier is vastly superior to any I’ve ever seen for concealed carry of a double stack magazine. Rides very flat to the body.
Is this the RCS carrier you’re talking about?
http://www.themalabarfront.com/product_info.php?products_id=1582
I’ve been using this lately with no problems very comfortable,
only person to question me was a co-worker when I wore it
shirt tucked in. He thought it was a pocket knife sheath.
I use it to hold my spare glock 19 mag.
I’ve always used a knife or leatherman pouch to carry a spare mag, since lots of people carry cool-guy tools. Hidden in plain sight. 
i put a mag in my left front pocket. it works.
I like the RCS mag holder and then I add the small triangles to the corners for pocket carry. Just do my HK P7 mag this way right now, actually put it in my jeans back pocket. Wife never caught it. The mag holder makes it look like a wallet, and the triangles held it in place in all the drills I ran.
For double stack mags, that may be a little thick. I was thinking about making some Kydex that fit the bottom of my front pocked and kept a mag indexed and orientated. Maybe even add a spot for a flashlight.
I was doing this until I had a random piece of plastic from some wrapper end up jammed between the first round and the mag lips. Took more to get it out than I thought. I don’t have the discipline (and two kids who hand me all kinds of stuff) to keep my pockets FOD free.
I’ve tested my glock 19 with a gum wrapper. no jams :o
i DO keep this pocket FOD free though honestly.
and i shoot ISPC matches from the pocket.
I use a Galco leather pocket carrier for my spare CCW mag:
Tomac
Yes, I got mine with the MD cut.
I actually carry my spare mag IWB. I may be the odd one out on that but its very comfortable and hidden. I just ordered a set of the IWB clips from PSS. The loops work too - tried them before.
LOL, I forgot that was you! Sorry. 
My buddy got one of these IWB mag holders from Comp Tac…he really likes it.
I would completely advise against pocket carrying magazines by themselves if you trust your life to them. I had a staple from when I went to the range that found its way into my magazine and if I didn’t screw up the press check and ejected the top round and then found the jam when I went to load back up again to find the jam I could of had a very bad day if I needed my gun. Perhaps a pocket holster for a mag would help.
As far as a OWB carrier I echo the sentiments about the raven set up. I have a double and it is by far the slimiest carrier that I have handled in my brief time I have had my CPL.
I have this and I swear by it. Combine it with an MTAC holster and you have a very effective and comfortable concealment system. Wish I had an MTAC 20 years ago. The advancements in gear over the last ten years have been spectacular.