I think using the upstairs tank's effluent as the feed for the downstairs tank does make sense.
The only thing that I think you'd have to worry about, in your plan, is the splashing caused by a 12' drop of water (or whatever it is). It won't really be a "slug" of water in that pipe, it's a mixture of water and air and so even if the splash could be minimized there will be a lot of bubbles to contend with.
The one thing that I felt was missing, when I tried Borneman surges, was that I didn't have enough height to work with to get the "crash" of water I was really hoping for. Maybe in your case you'd actually have some "wiggle room" to play around with a setup like that. The splashing from the water dropping could be contained within the surge tanks, and the tank itself would be fed, not by a continuous stream, but from a "splashing wave over here" and a "splashing wave over there" and so on.
Let me know if you'd like any help
I'd love to play with that kind of setup.
PS. Whatever did you do to get permission to get a tank upstairs? You're gonna have to coach me on that one.
