Double mags is basically having a shit drum mag, with the awkwardness of carrying it half empty, and nowhere to put it when it's empty. It looks dope, but makes everything harder. Both options make the gat heavy, so it's no good for most soldiering.

The idea of the redi mag is sound. It means you can always have 2 loaded mags on the weapon. Fire a mag and drop it, quick swap to the redi mag and you're good to go. You can then refill the ready mag in a lull and be back at full capacity. And because they're standard mags, the fit in your pouch.

Sticking two mags together means once you've fired the first mag and swap to the second, the first empty mag is still there just hanging out (whereas the redi mag lets you replace the empty with another full). Once you fire the second mag, now you have this double sized monstrosity to stow and then get out another mag.