make 2 doors.
set the angle on one door to be the reverse of the other door.
(like one has angle 90 and the other has angle -90)
give both doors this:
Key: targetname
value: MyFirstDoors
draw a brush using the trigger texture.
right click on it and make it a trigger_multiple.
select the trigger multiple and both doors.
press CTRL K
now when you touch that invisible trigger, both doors will open.
i left out some details but with some experimentation you will learn alot and it will become easy for you to get it right the first time when making double doors.