Hi Stephen,
In order to do things the way you outline above, from the rank table you'd also have to set everything up MONO, you'd also have to create 4 separate MONO audio groups in Hauptwerk for each of your 4 channels, an arrangement pretty much to me that would be a bit overly complicated for the instrument in question, and you may not like the results much either.
I have the St. Eucaire set, looking over the HC-15 and 18 speakers, and seeing you have a total of 4 channels to work with, if it were my options I'd probably be more inclined to do something like this:
Set up two separate stereo audio groups in Hauptwerk, one group for the 2- HC-15's (channels 1-2) and a second group for the 2- HC-18's (channels 3-4). Send both manuals (less any 16' stops) to the HC-15's channels 1-2, send the entire pedal division to the HC-18's channels 3-4, and also include any 16' on the manuals also to the HC-18's on channels 3-4.
Have the speakers (one HC-15 and one HC-18) stacked on top of each other on the left and right and get the stacks a good 6+ feet away from each other and see what you get, it should sound pretty good.
Have fun!
Marc