Is it possible to use the GRID control for a 1 to many form using a "compound" index expression?

This works...

Linkmaster = Customer
RecordSource = Invoice
RelationalExpr = custno
Invoice Index = custno
IndexExpr = custno

This doesn't...

Linkmaster = Customer
RecordSource = Invoice
RelationalExpr = custno
Invoice Index = custno
IndexExpr = custno + invno DESCEND

I want to do this to get the oldest invoices at the top of my GRID. Is there a better way to achieve what I want?

Thanks in advance,

Andrew