When you want to split or copy an order, on Orders page select order. Under Actions section you can select Split order or Copy order.
You can split the order when the order is PENDING for more products to arrive at the warehouse.
You can access new split order by clicking To back order.
When the needed information is in place, remember to save it to NEW state and the warehouse will start to collect the order.
Copy order will make a new draft copy which can be found on the order list page. The order reference remains the same except a index "_1" is added in the end.