Sure you don't have enough space for a lift?
It is tight, but I bet borderline doable to fit a Z4 and a 911 stacked in a normal ceiling height garage. I've seen many pics of similar setups; just looks like they have to put some kind of height marker on the lift to make sure they stop just short of putting the top car through the ceiling.
That was one of the reasons I moved a couple years ago... Two car, high ceiling garage that I could stick two lifts in if I wanted (do want one so I can park my DD inside also). Problem now, I don't have enough cash to fill the place up.
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If definitely not enough space for a lift, I'd just use the offsite parking year round and go trade cars every now and then.