Yes you can do that.
The way that the button triggers the encoder is by closing the circuit between the key that you are using for player1 and the ground.
In order for this to work you need to make it so that whichever button is pressed, that circuit gets closed.
If you wire from your encoder to one of the buttons like you normaly would and then run wires from the contacts on the switch to the same contacts on the second switch you should be set to go.
You are wiring them in parallel, so that if either button gets pressed, it closes the circuit and sends the signal.
I hope this helps, if I ramble it is because I am dozing off.
Later,
Brian