Or, just use the SQL CONCAT() function.....
$query2=("INSERT INTO query_results2 (Location1, Location2, Average_Income) VALUES (CONCAT('$drop_down_box_1', ',', '$drop_down_box_2'), '$area_1')");
That concatenates the variable $drop_down_box_1, and $drop_down_box_2, as well as a "," in between them. It reads from left to right, and you can add as much as you want. Each item to be added is separated by a comma. Strings are within quotes (obviously).
Hope that helps.