I think this is a "user" choice item.
Let the user optional choose to,
1) Load RAW only (primary or secondary dest.)
2) DNG Only (primary and/or secondary dest.)
3) Both RAW (Primary only) and DNG (Secondary only) (or vice versa).
Make the default RAW only.
Alternatively, have a post-processing step that executes on the Primary and/or Secondary dest after ingest. This way the original says and a DNG is added to the dest.
Just some thoughts.