One or two dynamo-powered headlights? It depends jhow fast you ride. With a DLumotec primary, I'm told the E-6 secondary provides good lighting at 12-14 km/h. But with a Lumotec or E-6 primary, the E-6 secondary starts to provide useful lighting at 15-18 km/h (i.e. both headlights better than one) and both reach full power at around 22 km/h. I don't find the secondary that useful in the city; it's a bit more useful in suburban territory and quite useful on dark roads, especially if going downhill.