Welcome to BigScreenBiz.com

We had two theatres in the chain I worked for that had been made into over/under theatres. They were both converted in the later 70's. One of them was a former Cinerama Theatre. It was converted into a tri-plex. The main theatre seated 1200 and the twins upstairs seated 500 each. They used the original booth from when it opened and the center Cinerama booth. The main and theatre #2 were both 70MM equipped.
And the other theatre was made into a twin. Both theatres the convertions were done very well, in the regards to sound problems and handling patrons at different levels inside the theatres.

In canada many of the older Odeons were twined as a balcony split
most have closed since
2 things to watch for is that often the ceiling drops over the balcony with structural steel behind it limiting the size of the upper screen
The second is the lower booth may have an issue with the balcony edge overhang limiting the height of the image remebering that the machine optical center is 48" above the floor elevated to clear heads
definitly get a good architect and cinema consultant