I tested Street Fighter 2, on a turbo grafx (using a convertor) with a 6 button controller than now has a turbo grafx cord. Functioned a bit flaky at first, however after checking some connections and contacts for the switches, all went well.
Making a tap that allows Turbo controllers on a Duo is pretty easy, Duo wire plugs into Turbo tap (did not test) however the plastic plug on the Duo Tap is a bit different, it would require some modification for the plug to fit.
Cool stuff.