Anyways, IMHO you may have a problem with your plan:
1. I don't think you will be able to do this w/ most regular HUs (w/o a very highend internal HU X-over). Your tweeters require very little power compared with the mids. The comp X-over attenuates the power going to the highs...you would lose this feature (IIRC: there are some add-on MBQ tweets/x-overs which have this feature). You would also lose the ability to level set between the tweets and mids.
2. You would then have to choose the right X-over freq and X-over slope. The external comp X-over already takes care of this. Some competitors (myself included) run seperate amps for the mids and hi's, but this requires a bit of tuning (amplitude, X-over pt, & phasing) to sound right.
3. IMHO, to do what you propose, you will need 2 2-channel amps and a good X-over
It would be easier to put the X-over in the door, use the existing speaker wire to power it, and run wire from the x-over to the hi's and mids. You need one 4 channel amp to run the whole car.
Crutchfield has the amp bypass harness which clearly marks which speaker is which. You can run your speaker wires from your amps to this and plug it into the existing harness behind the glove box.
My $.02
