What sort of differences should there be between these dropdowns? 

Different set of values?

Do you want then to use these selections in the workflow or just have them visible in the form?

If just visible, then the easiest way is to add (of course) both dropdowns to the repeating section, set their names, and then set formatting rules to them:

1. currentRowNumber({Self})>1 -> hide -- so that control with this rule will be visible only in the first row
2. currentRowNumber({Self})<=1 -> hide -- so that control with this rule will be visible in all rows but first.

And using similar rules you can add more fields and show them only in related rows.

Regards,

Tomasz