http://support.smartbear.com/viewarticle/56906/
Take a look at the last section, Adding Operations to an Existing Data-Driven Loop.
You were right about needing more than one variable in the loop. You will need one for each Excel sheet or table that you want to reference.
One other thing that might be causing you a problem - the sql query you mentioned for the combobox uses select top(1), which is only ever going to give you one record. You'll also have to loop through that db if you want to get more than one option from the table.
