Wiring for use of one light at a time is parallel wiring with the dynamo leads from both lights going to the dynamo and using the switches on the lights to select which is on. A fancier setup could be wired such that either light could be selected with a toggle switch on the handlebar. I do not believe that the SON, or any other Dyno hub, puts out enough current to run both lights at anywhere near full power when wired in parallel.

Series wiring, with one wire from each light tied together and the others attached to the dynamo as you described, MIGHT work at higher speeds but both lights must be on to complete the circuit. At lower speeds output would be reduced compared to a single light I suspect.

Have you emailed Supernova or Schmidt tech support to ask about their thoughts or suggestions on this combination?