SON produces two versions of the upside down Edelux 2 headlight,
One version has no switch, and the other has a switch function on it.
The version that does not have a switch function, does not support the use of a taillight.
The version that does have the switch function can be used with a dynamo tail light, however connection is slightly different compared to that of the normal SON Edelux 2 headlights.

You can tell the diffrence bewteen the two versions of the upside down light, as the version with switch function has both a switch and the cable to the dynamo pre installed. the version without switch function has no cables installed, and has no switch.
The only difference is that to connect a rear light, you need to use a 3 & 6mm ring terminal to connect to the front light (supplied in the box).

Below illustrates how to wire a rear light to the upside-down headlight with switch function.
You CANNOT use a tail light with the upside down Edelux without switch function, doing so will overload the taillight and permanently damage its electronics.