How can I change the POS behaviour so I can scan more than one discount voucher?

I have created some vouchers (barcodes) that apply 100% discount to the shopping cart line that is selected and that is working fine, I select the product line, scan the voucher and it applies the discount. When I select another line and scan another voucher, nothing happens. I'd like to be able to scan more than one voucher per POS Order.

Regards 

Jim