Merging Orders
Highlight two Orders on the main Orders screen, (the Modify Order button changes to Merge Orders) then Click Merge Orders.
Say Yes to the message after ensuring the correct Order Supplier will be retained. You can change the Supplier, if required, by right-clicking on an Order in the list and left-clicking Change Supplier.
To highlight 2 orders that are not next to each other, use the windows convention. Highlight one of the orders then press the <Ctrl> button on your keyboard. While holding this button down use your mouse to highlight the second order.