There were way more people who wanted an NES Classic than were able to get their hands on one. So far, Nintendo seems to have done a better job of meeting the demand for the SNES Classic (although we’ll see how that holds up over time). Either way, the overlap in the Venn diagram covering NES Classic owners and SNES Classic owners doesn’t represent a massive number of people so far.
Having said that, those lucky few will have plenty of controllers to go around. (While the NES Classic came with one gamepad, the SNES Classic includes two.)
The NES Classic and SNES Classic feature identical controller ports — it happens to be the same port that the Wii used — and we can confirm that the two consoles’ throwback gamepads are interchangeable. In other words: You can use an SNES Classic Controller to play games on an NES Classic, and you can play SNES Classic games with with an NES Classic Controller. The functionality was first reported by Select All.
Of course, the latter situation presents some problems. The NES controller only has a D-pad and two buttons, A and B, while the SNES gamepad has a D-pad and six buttons: four face buttons (A, B, X, Y) and two shoulder buttons (L and R). Obviously, any Super Nintendo game that uses more than two buttons will be difficult to play with an NES controller, if not impossible.
As for the inverse scenario — an NES Classic with an SNES Classic Controller — we tested Super Mario Bros. 3. In that game, the A button jumps and B runs. On an SNES Classic Controller, the Y button can double as B, and X can double as A. The same setup applies if you’re playing with a Wii Classic Controller or Classic Controller Pro hooked up to your NES Classic. (Those gamepads are also compatible with the SNES Classic.)