Here is a hidden staircase I made !

By having a part of the stairs that stays in the middle, i could use 2 flying machines go on opposite direction.

That basically makes two 3x2 hidden door.
You could probably make it 3x3 by having the flying machine go one block further.
This staircase use slabs so if you want to use stairs your will have to think about is a little.

Be careful where you place decorative blocks so to not block the path of the flying machine.