Jefferis,
It can be done but not within the extension interface, it is pretty easy if you are comfortable in code view (which I am sure you are) to add new search items to the code block and then join the 2 respective tables in the query as you would normally.
A couple of gotcha's if you have fields that are named the same in both tables you need to explicitly call them i.e. tbl_name.field rather than just field and you cannot use aliases in the search code block.
This is an example of how the code block would look in one of my admin sections
$WADbSearch1 = new FilterDef;
$WADbSearch1->initializeQueryBuilder("MYSQL","1");
//keyword array declarations
$KeyArr0 = array("tbl_orders.id");
$KeyArr1 = array("tbl_clients.fname", "tbl_clients.lname", "tbl_orders.shipfname", "tbl_orders.shiplname");
$KeyArr2 = array("tbl_orderdetails.title");
$KeyArr3 = array("tbl_orderdetails.details");
$KeyArr4 = array("tbl_orders.id");
$KeyArr5 = array("tbl_orderdetails.title");
$KeyArr6 = array("tbl_orderdetails.title");
$KeyArr7 = array("tbl_clients.fname", "tbl_clients.lname");
$KeyArr8 = array("tbl_clients.email");
//comparison list additions
$WADbSearch1->keywordComparison($KeyArr0,"".((isset($_GET["zoom_query"]))?$_GET["zoom_query"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr1,"".((isset($_GET["zoom_query"]))?$_GET["zoom_query"]:"") ."","OR","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr2,"".((isset($_GET["zoom_query"]))?$_GET["zoom_query"]:"") ."","OR","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr3,"".((isset($_GET["zoom_query"]))?$_GET["zoom_query"]:"") ."","OR","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr4,"".((isset($_GET["S_title"]))?$_GET["S_title"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr5,"".((isset($_GET["S_sku"]))?$_GET["S_sku"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr6,"".((isset($_GET["S_product"]))?$_GET["S_product"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr7,"".((isset($_GET["S_fname"]))?$_GET["S_fname"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->keywordComparison($KeyArr8,"".((isset($_GET["S_email"]))?$_GET["S_email"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
$WADbSearch1->addComparisonFromEdit("tbl_orders.dateinsert","S_dateinsert","AND","=",2);
$WADbSearch1->addComparisonFromEdit("tbl_orders.status","S_status","AND","=",0);
$WADbSearch1->addComparisonFromEdit("tbl_orders.client","S_client","AND","=",1);
The zoom_query call is a single multi table search text box
Hope that helps.