Hello there builders, I decided to show you an insane piece of my mind, the H-shaped stairs.
H-shaped stairs are basically pair of U-shaped stairs sharing the landing, and combine the best and worst of both U-shaped stairs and normal stairs. Their usefullnes is a bit limited by the fact they use quite a lot of place, but can be very useful in U-shaped or H-shaped houses where you'll have sims going between different corners and levels of the house, and give your hall feeling that there's lot of space. In my case, all stair endings lead to a front/back entrance (lower floor) or balcony (upper floor), while rooms are to sides of them.
Construction is same as U-shaped stairs, with the only difference in needing more space and double patience. You can connect the separate U-parts in 2 ways. My stairs are done in a way that is not forcing sims to really do U-turingn unless it's the shortest way, but swapping parts on one side would force them to, if you want.
Now, pictures of working H-shaped stairs. Due to how Sims3 works with displaying stuff on different levels, it may produce lots of weirdness as 2 of the screens will show.
Looking from upper level down, from above and sim's PoV:
Lower floor from above
Lower floor from side
As you can see, it's not easthetically perfect if you try to do screenshots in the lower room, but it's how the game renders stuff. Stairs are fully functional.
If you want these stairs to be used on multiple floors, you'd most likely only need to replace the "fence" on landing with wall, make next floor landing, and with the use of moveobject cheat, remove the walls after and place fence back. Said method works with U-stairs, but I haven't tested it with H-shaped ones.