How to change search operator for search form

Hi,

I have a search form that includes a checkbox set. The default search operator within the check box set is "AND". I would like to change this to an "OR". i.e. when Checkbox "a" and checkbox "b" are "TRUE" all persons where checkbox "A" "AND/OR" "B" are "TRUE" are part of the search results. How can I achieve this without changing the code.