Detention systems are just fancy storage areas. The only realistic modeling is a pond routing of a hydrograph into and out of your ponds.
In the Hydraflow Hydrograph product - you can model your site, route into the underground pond, and then into the other detention pond. You do indicate that the underground detention pond is about the same elevation of the storage provided by the excavated pond. I would assume that they would function together and this would be considered a interconnected pond.
Hydrographs only allows a one-way flow - so be sure you are not discharging anything other that your underground detention system into the pond.
Matthew Anderson, PE
Joseph A Schudt & Associates
Matthew Anderson, PE CFM
Product Manager
Autodesk (Innovyze)