Recommendations for a wireless keyboard for a Dell inspiron 13 700 2in1 and asus ux360C notebook PC?

If both systems have Bluetooth, that's the simplest way, as you won't need a separate receiver that occupies a USB port (and would need to be physically moved from system to system each time you want to switch between the systems).

Otherwise, any standard wireless keyboard will work.  If you want to buy a second receiver to allow connection to both systems without physically removing the receiver from one and connecting it to the other, some manufacturers (Logitech, for one) are better than others at supporting the purchase of a second receiver.  Others likely won't sell you just a second receiver without buying a second keyboard.