Need more info to help. Having more speakers than channels doesn't necessarily mean you need a x-over. What type of speakers are they? Are they components or full range (coaxial / triaxial)? Or are you trying to add an amp to a stock BMW system?
(Edit): Okay, I just saw your other post ("Help with install..??"), and now I understand what you're doing. But can you give me the model numbers of the Nak amp and MB Quart speakers, and/or some pics if possible? Low and high crossover inputs doesn't sound right to me, but if I can see a photo or pull the tech info off the web, I can probably help you out with this.