A reference selector allows you to only select 1 object to reference. If you need to select multiple objects and then perfrom some filtering on a second selector you’ll need to adjust your domain model accordingly and use the reference set selector. Then the filtering can be done in the microflow being used as the source for the second selector e.g.
For more information on the reference set selector see: https://docs.mendix.com/refguide/reference-set-selector