Best practice for associative machining between products

I have a cabinet that is 120" tall with a lower carcass @ 96" and an upper @ 24". I have a single door that spans both cabinets with 5 hinges total, 3 below, 2 above. If the upper and lower are listed as separate products, what is the best practice for associating the door machining with both of the cabinets. Note that I tried creating a product with both upper and lower carcass and the door, however, the upper and lower will ship separately so the shop wants them as separate items. Any thoughts on how to please MV and my shop?

PS anyone else not able to post here via google chrome? When I try to post from Chrome, there is no Description field so I couldn't enter a description,