There is currently no way to use DatasetModels to provide a strong typed filter argument for multiple tables.
Best you can do is probably this here:
DEFINE VARIABLE oOrder AS OrderDatasetModel NO-UNDO .
oOrder = NEW OrderDatasetModel() .
oOrder:RetrieveData("eOrder,eOrderLine",
"for each eOrder where eOrder.Custnum = 1" + CHR (1) +
"for each eOrderLine where eOrderLine.LineNum = 2") .
oOrder:UseDatasetQueries() .
DO WHILE oOrder:Order:Available:
IF oOrder:OrderLine:Available THEN
MESSAGE oOrder:Order:CustNum SKIP
oOrder:Order:Ordernum SKIP (2)
oOrder:OrderLine:Ordernum SKIP
oOrder:OrderLine:Linenum
VIEW-AS ALERT-BOX.
ELSE
MESSAGE oOrder:Order:CustNum SKIP
oOrder:Order:Ordernum SKIP
VIEW-AS ALERT-BOX.
oOrder:Order:GetNext().
END.