Minecraft: Quick and Easy Double Door Opening with Buttons
Here's how to make two doors open at once using buttons at eye-level and a tiny bit of redstone!
Materials: Two doors, four buttons, some building blocks, some redstone dust (minimum 4).
There are buttons beside each door on the inside and the outside, and you can use whichever button is most convenient for you, but all open both doors. You could also simply add on buttons to the lower sides of the doors and get just the door that's next to each lower button to open, if you really want that fine-grain control over your door opening.