Using these multiple criteria is no different than using a single criteria in the DataAssist Search server behavior, you can just add as many as you need for the search you are performing.
The trick is that you need to have the recordset on the results page setup to select all of the possible records you would like to search through. Then you use your new criteria that you add to the DataAssist Search server behavior to filter this recordset so that it only returns the desired results.
Do you have the recordset crafted so that it will return all the results? If not this is the first place to start. Next you would add in the additional search criteria to filter this recordset. Please post back with any questions that you have about any part of this.