A field (perhaps in a repeater) is linked to a data collection. Right now I can only select the collection associated with the page. I should be able to select a second collection and specify the field in my page / repeater collection which contains the key field for the second collection. If there is a way to do this I’ve not found it yet.
Use reference fields to link items between different collections. Later you will be able to connect the repeater to data coming from both collections.
See Adding a Reference Field to a Database Collection | Help Center | Wix.com