From my reading and experience the brighter German street legal lights such as the B&M Lumotec IQ Cyo, Schmidt Edelux and Inoled Inolight Extreme draw between 2.4 and 3 watts. German dynamo lighting regulations specify that these lights must reach full brightness at quite low speeds which limits their allowed power draw. They also have strict regulations on acceptable beam shape for all street legal bike lights.

Several more powerful dynamo LED lights are on the market which are not German street legal. These include the Supernova lights, the Light On! Dynolight and the British Solidlights unit. These do not reach full output until higher speeds. The Solidlights light is listed as requiring 6 watts input by the manufacturer. The Supernova E3 standard (not the Triple) is noted as drawing .4 watt too much power for German road approval even thought the Asymmetric version now has a German street legal beam shape. Per their web site the Light On! reaches full output at 18 to 20 MPH. Per an email from Supernova their E3 Triple reaches full output at about 25 MPH. I would not expect these to be suitable for multiple installation if hooked up in series for simultaneous use.

Per Peter White Cycles web site none of the current commercial LED lights are really designed for multiple use. If hooked up in parallel, with a selection switch, the higher power ones could have two installed but not run simultaneously. The lower power lights could presumably be run in series like the old bulb lights but like them would not reach full light output until substantially higher speeds are reached.

My own choice for a dual LED light installation would be two Supernova E3 Lights set up in paralllel with a selection switch. One would be the asymmetric beam E3 for in town use, effectively giving a low beam, and the other would be the E3 Symmetrical or E3 Triple for out of town and high speed night use.