VR,
I think you mean parallel. My setup for instance has one set of wires coming off the battery, P and N. Then there is a Y off the P and Y off the N with one each P and N going to the taillight and one each to the headlight. If you had two headlights you would have another set of Ys, even if you wanted them switched on and off together. Series would have one continuous loop from battery to light to light back to battery with no Ys. I'm pretty sure the former is the preferred method. That's pretty much how houses are wired.
Regards,
Raymond