Portal – Tutorial – Multiple Buttons For One Door
... 
This tutorial will detail the basics of getting a door to open only when two (or more) buttons are activated.
Door, buttons and triggers
Create your doorway and two buttons by following the tutorials Floor button and Test Chamber Door.
Setting the buttons to open the door
Create a math_counter entity, and put it near your door. Set the following properties:
Property Name Value Name door_counter Maximum Legal Value 2
For the Maximum Legal Value key value, you can enter whatever number you need.
Now go to the Outputs tab and set the following outputs:
My Output Target Entity Target Input Parameter Delay Only Once 
OnHitMax door_left Open <none> 0.00 No OnHitMax door_right Open <none> 0.00 No
Now go to each of your floor_button_door brush entities and set these outputs:
My Output Target Entity Target Input Parameter Delay Only Once OnOpen door_counter Add 1 0.00 No OnClose door_counter Subtract 1 0.00 No OnClose door_left Close <none> 0.00 No OnClose door_right Close <none> 0.00 No
