What would be a safe minimum vertical separation distance between passenger-carrying multicopter aircraft? There is increasing talk about employing passenger-carrying multicopters ("taxi drones") to extend urban mobility into the third dimension. These aircraft will have MTOMs in the range of several hundred kgs.

In case this catches on on a larger scale, airspace use will increase considerably compared to the current occasional helicopter and sight-seeing flight.

For safety reasons, presumably a certain minimum vertical separation will be required due to the downwash from the rotors.  

Is there any information available about the current thinking on required minimum vertical separation between manned/passenger-carrying multicopters? Also, will there be any difference for this requirement between hovering and cruise flight modes?
The downwash is directly related to total lift; more rotors just means each rotor is producing a smaller fraction of the total, and the end result should be no different for separation purposes than for one or two rotors.

There are a lot of obstacles to mass adoption of personal aircraft, but the exact lift mechanism is arguably the smallest.