Chamberlain 045ACT Opener Button
The pushbutton is an N.O. (normally open) switch.  When you press it, the circuit is closed and the opener starts its cycle.  It's easy to add a 2nd pushbutton by wiring it in PARALLEL to the first one.  That way EITHER one, acting alone can control the door.

For a diagram of this see the 'Switches in Parallel' diagram at:

Switches In Parallel