If you cut off the key from one, it can be used as a remote only; the other key you keep in the car, uncut, could still function fully.
Another option would be to get a flip key:
2011 13 Ford Mustang ALIN1 Flip Chip KEY Keyless Entry Transmitter Click Remote | eBay
After you get it programmed to open the doors and/or start the engine, you can remove the key portion from it.
I have one, but have stopped using it because the buttons are so sensitive that having it in my pants pocket while sitting has set off the panic alarm.
in Huntington Beach, CA, USA
2013 Mustang GT base, auto, convertible, DIBM, FRPP ProCal tune M-9603-MGTB with an aFe Pro Dry S drop-in air filter 30-80179, Conceptual Polymer glass oil catch can CP04PA011D.