You don't need a dual-beam bulb if you have a projector headlight and here's why. There's three different types of projectors - bi-xenon, bi-halogen, and bi-LED. All of these use a single light source. If you've ever noticed that distinctive line to where there's light and then no light whatsoever, this is due to this cutoff shield. And how this works, is when the low beams are on, this cutoff shield is up, and it is not allowing glare or anything extra to be blinding oncoming traffic.
What then happens is your vehicle will send power to this actuator. This actuator is what controls this cutoff shield. Once this actuator gets power, the cutoff shield will drop down, then revealing the full high beam. Once you turn the high beams off, the cutoff shield will go right back up.
Now, whether you have a bi-xenon, or a bi-halogen projector, the process is the exact same. The only difference is the lighting source. Bi-xenon, you'll have an HID bulb. Bi-halogen, you'll have a typical halogen bulb. No matter what, though, it is a single filament HID bulb and a single filament halogen bulb, not a dual filament bulb.
Now for that bi-LED projector, there's not much of a difference. The one main difference here is there's no removable bulb. Let me show you how the bi-LED projector works. It's practically the same as the bi-xenon and bi-halogen. Inside here we have the LED chip set. Same way as before, those LEDs will shine off the chrome inside and shine outwards of the projector.
The cutoff shield is what will actually dictate the low and the high beam. No power going to this actuator here, cutoff shield is up, thus giving you that nice crisp cutoff line. As soon as this actuator gets power, cutoff shield will drop down, engaging the full high beam. The power source does not change. You're not getting more power, more LED chips, anything like that. You're just getting more light.
So when you ask me, "Do I need a dual beam bulb inside of my projector?" you simply do not. You just need a single filament light source, whether it's an LED bulb, a HID bulb, or a halogen bulb, and then from there, the projector will take over. If you have any questions for me, feel free to give me a call or shoot me an email. I'd love to help you out here.