Since I run Magpul magazines, I was quite contented with the Magpul couplers.
But the addition of a magwell required a coupler that spaced out the two magazines just a little further to clear the funnel. The Arrendondo coupler will clear the larger magwell and also fit G.I. magazines.
http://www.midwayusa.com/Product/255862/arredondo-magazine-coupler-ar-15-ruger-mini-14-polymer-black