Merging invoices

We sometimes get Purchase orders with combined totals from different quotations. For instance: 1 quoataion for repair truck A= £ 20 and another Quotation for truck B = £ 30 then Purchase order comes with Repair truck A and B = £ 50. Therefore to save space, time and to avoid confusion when accounting for invoice numbers against purchase orders, it will be ideal to have one invoice matching the purchase order. so for this to work there should be a function to either merge quotations/tax invoices.