TXGunGeek has a review of Springer Precision‘s Firebird PMAG Coupler.
I’ll let Mr. Gun Geek’s review do most of the talking. I’ve seen and gotten to play with the very same couplers he reviewed. They weren’t too bad. I was concerned about how they’d affect balance of the rifle in your hand, and they really didn’t have an adverse affect that I could feel.
Will I get them? Probably not, only because I’ve been looking for something else. I want to have more ammo on the rifle but not necessarily coupled together. Something like the Boonie Packer Redi-Mag may fit the bill. Yeah, TXGunGeek has an older version of the Redi-Mag on his AR. I was considering the Redi-Mag long ago, and being able to play with TXGunGeek’s has helped prod me in that direction, but I still haven’t made my decision.
That said, Springer’s Firebird PMAG Coupler is still a pretty cool thing. If that’s the sort of solution you’re after, like all things Springer Precision does it’s high quality and you should be satisfied.
Stuff From Hsoi 
If they were more like $10, I’d be all over it.
After having dealt with CAA’s failed attempt at a stock magazine holder, I am definitely going to have to look into those Red-Mags… personally, I like the idea of an expended magazine dropping free instead of sticking around for the ride, but that is just me :).
I hear you on that.