One handy feature of the typical dynamo is that they produced a fairly constant short circuit current of 0.5A over the usual range of speeds. For the sake of the rest of the audience, this means that if you short out the dynamo's output, you'll get 0.5A or so. The resistance of the incandescent bulbs was chosen to be nearly a short circuit; 12 ohms. In the days of incandescent bulbs, which are extremely sensitive to variations in applied voltage or current, this semi-regulation provided fairly long bulb lifetimes and steady illumination levels.
The downside is that the bulb nearly short circuits the dynamo, causing higher losses than would be needed if the dynamo was operated somewhere near the middle of its voltage-current curve.
People have been wiring two incandescent lights in series as a method to increase the light output for a long time. Peter White used to sell a wire harness for this purpose, I believe.
For these reasons, running two lights in parallel is almost guaranteed to not increase the total light output. The lights would just be sharing the half amp.

Steve in Peoria