I don't think you are going to be happy with anything less than 12" between levels, and will probably be much happier with 18". With less than 12", you won't even be able to see the lower level, much less reach in to construct or maintain or even re-rail equipment.
On the double-deck layouts I have seen, vertical deck separation is a compromise between desired layout height, viewing angles, and depth of layout. Often the upper level has reduced depth to give a better viewing angle to the lower level.
I strongly urge you to build a full-size mock up with cardboard and wood to see how the sight lines and access will work in your particular situation.
I used a full size mock up with cardboard boxes to test the practicality of 24" aisles with a 50" high layout. Needless to say, I was not pleased with my findings. It was barely adequate, but it certainly wasn't comfortable. I could see a lot of knocks to the layout edges and details. The result is certainly going to further restrict how much layout I can have in my space, but at least I learned before building with expensive lumber and hours of handlaid track.
I think you are going to find that the proposed 2 level design in the space you have simply won't work. There is a Model Railroader published design that contains the word Laurel in it (I'm having a senior moment and can't remember the rest of the name). It fits a 5x8ft space, and is also published in 48 Top Notch Track Plans. It has an oval with a small interchange yard and turntable at the base level, and a branch that occupies the center of the oval and terminates in a wye.
my thoughts, your choices