InfoPath doesn't care if the secondary data source has one or a hundred rows - the XPath to the field is going to return the first node, unless you've specified with a filter which row you want.
So, let's say I have a secondary data source called "Cities". It has a field called City.
The XPath looks like:
Cities/Row/City
Row is repeating.
My main data source field is called SelectedCity. It is bound to a dropdown control.
The dropdown uses my Cities data source to provide the values to the dropdown.
If I set the default value for SelectedCity (my main data source field) to my secondary data source field:
Cities/Row/City
SelectedCity will always default to the first entry in that data source.
Does this make sense? If not, I can put together a quick sample form.