Salam
Sorry I don't know about Benz
for HID, to put it simply, HID light is a high intensity discharge light which has xenon gas filled in the tubes. this gas gets heated and discharges high intensity light.
therer is used only one tube in head lights of cars like City, Lancer. This tube has two separate filaments in it which work for low beam and high beam.
Bi-xenon actually has two xenon tubes fitted together which work as hi and low beams. you require 4 ballasts for it

bi-xenon available in the local market is actually retractable HID tube that has both hi and low beams in single xenon tube. usually in hi/low HID, one beam has hallogen bulb whereas other beam is HID as seen below:

whereas Bi-xenon has single HID tube working as both hi/low beams. it is controlled by a motor in the tube's holder which moves the tube to give hi and Low beam. this tube looks like this:

I have just installed a Bi-xenon kit in my City.