Two Foreign Keys??? (1 Viewer)

Better to use an Autonumber key field and if you want to store the Sales Order No as text,use a separate field

tblSalesOrders
SalesOrderID Autonumber
SalesOrderNo Short text
CustomerID Short text
OrderDate Date/time
[Other fields]

tblSalesOrderLines
LineNoID Autonumber
SalesOrderID Number
LineNo Number
ItemCodeID Number
[Other fields]

Use a query to join the two tables using the fields SalesOrderNo, LineNo (link on SalesOrderID) and include any other fields you want.