).In regard to the headroom to stack, it will of course depend on the height of your cars, but there are several other things to consider. In my case, I have 130" at the door opening. You need to start subtracting from this figure.
My garage floor has grade so it drains toward the door. It's not much but you could lose up to 3" to the ceiling depending on how far upgrade you go.
At 9', if you have a conventional overhead door, you'll likely need to get rid of any trolley type door lift and high lift the door close to your ceiling. You'll need to consider the entire thickness of your door, any reinforcement rails on the backside, plus several more inches for clearance. Subtract this from your ceiling height.
The thickness of the deck varies on lifts but BYB is 4". Subtract again.
Whatever you have left is what you have to accomodate the height of both cars plus clearance between your top car and ceiling and bottom car and lift deck.
When I do all this at 130" I concluded that I could get my Pcar (mine is 42") raised such that the deck height was at least 75" with a few inches clearance. Add another 6" with the door down. Besides stacking, I figured this was adequate room to work standing under the car.
Good luck.