I have a Shutter Precision PD8 hub, and a B&M headlight. I happen to have a spare B&M headlight of a different model in my parts bin. I'm thinking about putting the second light on just to have extra light, and potentially to aim it lower so I can see close-up better. Can I just patch the second light in? And if I do, should I put it electrically parallel or series? And will it be any brighter at all, or will each light just be proportionally dimmer?
Most bike dynamos produce roughly constant current, so if you ride fast enough, doubling the resistance by wiring two lights in series gives 12v, neatly split between the lights. The classic setup in old days was two lights in series and switch to short circuit one when riding slowly, so that the other would get the full voltage.
Some new LED lights designed for dynamo can take advantage of this feature even with single light, so it would make sense to not add any current limiting to new models. The only way to know if it works for any specific model is to try.