Render_complex_sql 3rd parameter

Hi all,
I’m trying to display a combo with 2 columns using render_complex_sql

The 3rd parameter of this function should take “a comma separated list of data fields”… however when I write “field1, field2” for the 3rd parameter I always see only field1 in the combo’s list!

Is it possible to see a PHP sample code to make a two-column combo?

Thank you very much,
Marco