my guess is that if its a single floor plate that's "permanently attached" that would make the mags connected and then it'll be considered one mag, if its something like a mag coupler, the mags each would still be independent since its not permanently attached, for ex. 2 AR mags with a mag coupler or duct taped together, it's still 2 mags. mako group also made the pentagon mag coupler for the AR to put 5 10 rounders together for restricted states, each mag still individual 10 rounds. however if you have 2 single stack mags that are individually inserted, and also each fire control group as well is independent from another, i would guess its legal since each mag is only single stack and its below 10 rounds mag limit. just my educated guess, I'm not a lawyer!