I organized my 2 staircases - upper and lower - as areas with motion sensors at the top and bottom of each one.
Now I’ve created floors and assigned rooms to floors, but what “floor” should a staircase be on since it is on 2 floors?
Should I leave them unassigned or maybe create a “lower upper staircase” and “upper upper staircase” area , for example. And then upper upper staircase could be on the 2nd floor, and lower upper staircase could be on the 1st floor.
I doubt there is a right or wrong way to do this. The answer should probably be: Do it the way that seems most intuitive and logical to you and any other folks who use HA in your house.
In 3D images the staircase belongs to the floor it consumes, my basement staircase consumes space in my basement (from floor to ceiling) and does not consume any space on my first floor. So I consider that staircase to be in my basement.
I struggled with this one for a bit too, this way of thinking about it just made sense to me.
To me too. I have a staircase with a right-angle turn. The lower part consumes space on the ground floor, the top part consumes space on the first floor, and that’s how it appears on my (2D) floor plans. Which is no help at all… 
That perspective makes sense to me. Now what about my fireman pole from the 2nd floor to the 1st floor? j/k Solved!
Fireman poles are so dam cool they deserve their own floor IMO 