Running two headlights in series was a fairly well known technique back in the era of incandescent headlights. Compared to modern LED lights, they were truly weak, so any advantage or improvement was worthwhile. One virtue of incandescent bulbs was that they were designed to be electrically identical. When operating, they were nominally 12 ohms in resistance, resulting in about 6VAC out of the dynamo.

Modern LED headlights don't seem to be designed to look like a 12 ohm resistance, so it's harder to know how it will work with two wired in series. In addition, some dynamos are intended to be used with lower power headlights, so this just makes it harder to know if they could power two lights in series.
My guess is that if you wire two identical LED headlights in series, there's a good chance that it will work. Presumably one headlight will have the taillight attached. The other headlight will have to be isolated from the frame... many are designed to have one terminal connected to the frame via the mounting hardware. If both lights were to have their ground terminals connected together through the frame, I think that one of them wouldn't get any power.

Be aware that operating two headlights in series will result in less light when riding at low speeds. Most schemes like this include a switch to short out one of the headlights at low speeds.
I've made a LED headlight that was equivalent to wiring two headlights in series. It included a circuit that monitored the dynamo speed and shorted out one half of the LEDs when below 14mph. The increased light when operating above 14mph was noticeable, but wasn't as much as I hoped for. The next headlight that I built was the equivalent of one headlight.... the extra weight and size of the extra LEDs, optics, etc.,, just wasn't worth the small additional light.

Steve in Peoria