You should read through the users guide , it has ever single answer you are looking for
It is on the apogee website , so you can download it and take your time with it.
X-HD Users Guide
By the looks of it you should connect your core card to the primary port of the AD16x , then connect the secondary port of the AD16x to the primary port of the DA16x. This is how it is laid out on page 10.
Good luck!