It sounds like what you want to do is entirely normal. Just place the dropdown in your repeating section, binding it to the node of your choice, and then open up the dropdown's properties.
- Under List box entries select Look up values from an external data source.
- Select the data source you want to use to populate the dropdown.
- For Entries, select the repeating node in your secondary data source, and select Display and Value fields underneath (or on) this repeating field.
If I've understood your question correctly, that should do it!