Tricky! As far as I know, you can only filter on one email.
Maybe you should make two reports - an agent report which can be filtered by agent email, and then a retailer report which can be filtered by retail email?
If you want to use filter by email, you’ll need to choose one email field, unfortunately. The easiest solution (as Sam said) is to create two copies of the report and filter by email differently on each one.
If your data is in BQ, you can use the email parameter (@DS_USER_EMAIL) in a custom query which will allow you to compare it to both email fields. Once upon a time this parameter was available in Looker Studio directly, but not anymore.