Mapping two buttons press as a 3rd button
Sorry, not sure what do you mean by combining them with diodes. Are you talking about multiplexing? 


If your buttons are connected between signal and ground (with a pull-up) you can often use a set of diodes to make a combined button:







simulate this circuit &ndash; Schematic created using CircuitLab

This does not work if your buttons are in a 'button scan matrix'.