This is how to make a simple push button->door.
First make the door (select brush, make it a func_door). Set up its properties if you want (speed, direction, etc). Then make a brush for the button somewhere. Put it next to a wall, select it, and make it a func_button. Set its direction (the direction it moves when pushed), hit escape.
Now select the button brush, THEN the door brush, so that both are selected. The button must be selected before the door. Hit control-K. This links the button to the door. That's all it takes for a simple button->door.
If you want two doors to open at the same time when one button is pushed, do the same thing as before but make the two doors seperately (select one, func_door, deselect, select other, func_door). When you link the button, only link it to ONE door, not both. Now select the door you linked, open up the brush's properties (N key), add a key named "team", and give it a name (door1 or something, just make it unique). Add a team key with the same name to the other door. Now whenever you hit the button, it will activate the linked door along with all other doors with the same team name.