Well, obviously it can play both systems at once, but most systems use their own unique flimsy controllers, so you're stuck using their brand instead of an official S/NES one. Aside from that, the code in these systems are only emulators, not official Nintendo code, so there will probably be some compatibility issues.
Well from my experiences with clone systems, trust me you want the originals it's just not the same otherwise the sound and other things are just a bit off.
You can find the original systems them self's rather cheap if you hunt around(pawn shops, yard sells, thrift/second hand shops such as goodwill).
Now i'm not saying clones are a bad option but the originals are always a better option though not always cheaper, except in my case i paid a $1 for the SNES and found a Sharp 19SV111 tv (that's a rare tv with a NES built into it)on the side of the road.
I just started back with NES collecting. Starting from stratch, I'm up to a top-loader w/ 2 dog bones, a toaster with a new 72 pin, and 47 games and still going. Not gonna go for a full 677 set, but I'll probably have around 200 NES games when it's all said and done.
The Retro Duo is one of the better clones. I used one for a bit. It's the only one with s-video out and it seems to have at least basic Super FX support(I was able to play a PAL copy of Starfox on it). And it uses standard SNES control ports.
I don't care if the sound is off by 5% as some of the clones are. It's when you get into "really far off" like the AtGames Genesis clones that I run away. That, and AtGames prices their crap so high, it's cheaper to get real hardware anyway.
Unfortunately, Cincinnati is packed full of people who think their old NES is worth $50 bare minimum($100 if it's got a new pin connector and 5 crap games) and rarely will I see a Super NES below $40. I'll gladly buy a clone in that case.