Quote fromQuote fromJust place a button beside the door on either side of the wall.
Wow? That simple. I tried doing that with levers and only one lever would open it.
Levers work, but it'll be of the form: if either or both levers are "on", the door is open, else if both levers are "off", the door is closed.
To use the levers so that flipping one from either side changes the door's state, you'd have to use a 2-input XOR gate, the inputs each being one of the levers.
Otherwise, maybe one of the levers wasn't powering a block directly adjacent to a door?