Sure.
The coin mechanism has a momentary switch at the bottom of it which the coin trips as it goes by. (so it's basically a button you push with a coin)
If you take the two wires to that momentary switch and extend them to another momentary switch (button), it will also work to give one credit.
If you want to put a toggle between them so that only one or the other works, just bring the wire to the PC board (game board or encoder, whichever) to the center pole of the toggle switch, and the wire to the coin mech on one side and the wire to the button on the other. Then you can toggle between them.