Hi snow.snow,
You can set the default value of the dropdown list box to the item in your secondary data connection. That will display the first item in the list. The formula should look something like:
xdXDocument:GetDOM("TestList")/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW[1]/d:Title
Where "TestList" is your secondary data connection and d:Title is the field used to display the items in the dropdown. While [1] is manually added in the formula.