Pretty much any Bluetooth keyboard will work with either iPad. There's no real difference between themas far as keyboards are concerned, other than the physical dimensions. Keyboards that are specifically made for iPads may have an extra function key or two that will work with the iPad, though in many cases this is just a matter of labeling. The function key will be there, but labeled as something else on a standard keyboard.
If you're looking for a case/keyboard that will fit both, that would be tough. You'd probably have to settle for a folio style case keyboard where the iPad straps into one side, and accept a less than optimal fit for one of the devices.