Typically orders with items that are either unreleased or on back-order will be sent when the out of stock items become available. If you are happy to wait for the complete order to arrive, nothing else needs to be done.
However, if you wish for the in-stock items to arrive as soon as possible, we recommend placing them as separate orders. If you have already placed an order, you can either request a cancellation of the pre-order item(s), or have them split to a separate order.
Please note that additional fees will apply in the event of a split.