The DL251 untilizes 3 x AES50 ports. Therefore 3 x AES50 ports from the PRO2 will be connected to one of the DN9680's. The second DN9680 will have the same 3 x AES50 ports connected to the DL251. The connection between the DN9680's can be either CAT5e or Fibre. The Fibre will be either multi or single mode depending on the option installed.
The ideal way to set up the system, would be to have the PRO2 as the Master, with the DL251 slaving from it. The DN9680 connected to the PR02 should be set up to clock from 'External AES50' and the DN9680 connected to the DL251 should be set up as 'External Snake'.