from
http://faqlight.carpassion.info/headlamp-harness.html
My HID bulbs turns off when I am using my high beams
Most halogen headlights (not only dual filaments) are designed to turn off low beams when high beams are turned on. This is more of a engine overheating precautionary thing from the manufacturers than a result of a lighting law cotaining of too bright total headlamps.
In theory, using a diode from each of the high and low beam harness in series to trig relay so that low beams gets power even if low beams turn off will prevent this. It works great if you have halogen low beams, but not for HID low beam system. The downside is that if you are driving during the day with your HID low beams off, and do a couple of flashes with your high beam, you will notice that your HID system will be working like crazy. Startup time for HID system is up to 10 seconds, so flashing like this will only decrease life of your HID bulbs.
What a diode is:
Wiring diagram including a diode: