Is there a benefit in terms of Rushmore in creating compound indices for free tables?

For example, when we extract invoice headers, part of the SQL statement might contain "where cCustomer = 'ALLAN' and cInvoiceNo = '123456'".
Should there be an index on 'cCustomer + cInvoiceNo' as well as 'cCustomer' and 'cInvoicNo'?

TIA.