Solution 1 - You can give all furniture objects any desired elevation.
So, insert a flat box as a halfway floor and give it the desired elevation.
Then, give the second staircase the same elevation as the halfway box.

Solution 2 - If you want to use a floor, do as follows:
- insert a new level;
- give that level an elevation that's half the height of your room;
- on the bottom level insert the first stairs;
- draw a room without a ceiling on the new level;
- staircase two goes on the halfway level and reaches the floor of the next floor.

Success.