Regular HID Xenon headlight systems use a Xenon bulb for low-beam operations and traditional, high-wattage, halogen bulb as the high-beam. This is where Bi-Xenon comes in, Bi-Xenon light systems use the Xenon bulb for both high and low beam operations. It's a common misconception that a Bi-Xenon system has two Xenon bulbs per housing, this is false. A Bi-Xenon light housing uses a single bulb with a mechanical movement to alter the amount of light outputted by the headlight. Usually, Bi-Xenon housings use a shield that covers part of the bulb for low-beam operations, then, when high-beam is turned on, the shield is removed exposing the full light bulb.
Not sure what bulb your car is using? Try this Automotive Bulb Reference
site to get the proper bulb information for your car.