http://www.mayflash.com/pc/pc044/pc044.htm
i bought one off of amazon quite some time ago but only this past week have i had a chance to test it with two controllers at the same time. everyone complains that only the b button and maybe the directional pad work on the 2nd player's controller when using both pads.
mayflash says on their official site to use official pads, since 3rd party accessories might not work. considering they are based in hong kong, it's likely that they are using/recommending the japanese super famicom controllers - which is what i got to work on mine.
at this time i don't have any other super nes/famicom controllers to test this with, but i'm very satisfied with the performance of mine! i'm using snes9x under mac OS 10.6.4 (snow leopard), and will try it on windows when i get a chance