For IPSC, I use two singles and a double, as it gives me flexibility to accommodate belt loops and such, and keep the mags where I want them. When I'm carrying only two, I like to know that the mags will both be in the same place, and that they won't tilt or otherwise move in relation to each other.
Double open top mag pouches. Galco for the S&W 9mm mags, Fobus for the H&K 40 mags and Bianchi(sp?) for the 45 mags. When I shoot USPSA or IPSC I have a few of the blade-tech singles(qty 5). All mag pouches are open top.
Have both KyTac and Cen-DEx singles and prefer the Cen-Dex.
Being able to position ea. holder precisely is better than a double, IMO.
The Cen-Dex pouches are very robust and built like Tanks yet very fast. Good for the Street too....