I think you can have more than one DataAssist Search server behavior on the page at once but this should not be necessary if you are searching through the same recordset. The current error you are getting looks to be a code order issue, if the class if referenced before it is defined you will get this error. This class should be referenced in the include files that should be at the top of the page. If the proper include files are above this code the error should not occur.
To set this up so you do it with a single Search Server Behavior you need to make sure that you have the form fields in place first. Then you can edit the server behavior and just add in the additional criteria. When you add them make sure to change the separator to OR, then select the column it's type, the type of comparison to make, then select the form element that this value will be coming from. This should allow you to create a single Search server behavior that does all of this.