Multiple bikes represent a challenge for lights? Why isn't it a challenge for tires? Or shifters? Or seats? Or wheelsets? Or suspension systems? Or anything else? I'm currently running more than five bikes myself and the lighting system is pretty much the cheapest component I have to consider. Personally if I needed permanent lighting systems on any bike and generators were the choice - they've all have generators. If I wanted battery systems - they'd all have their own batteries and lights. I occasionally lend stuff out for group rides and it means everyone is properly equipped.