You need a relationship that matches by DistributorID.
Evetn Records::DistributorID = DistributorsByID::DistributorID
Then you can format Events::DistributorID with a value list of DistributorID's and DistributorNames from the DIstributor table. Then your conditional value list of RepID's and RepNames can be set up to list values from DistributorsByID with Include only related values starting from Event Records.
Note: I called the table occurrence DistributorsByID to distinguish if from your current relationship that matches by RepID. By duplicating the table occurrence for Distributors, you can define two different relationships: 1 that matches by RepID and one that Matches by DistributorID.