Splitting shipping options based on inventory levels (In stock ships now - out of stock ships later)

[ajizzle]

Hello,

We have a single location shipping products around Australia (and some around the world)

Wondering if anyone has found a shopify solution which allows the customer to see options of shipping in stock and out of stock items to either

a) Ship all together when in stock at X cost to the customer.
b) Ship in stock items now for X cost to the customer and out of stock items as a seperate shipment later for Y cost to the customer 

So essentially split the order into two seperate orders if the customer selects option b

Right now i have a pop up that states the following as someone tries to add a an out of stock item to the cart

'X item is currently out of stock. This will be back-ordered and sent when available
WARNING: If you submit an order containing 'in stock' and 'out of stock' products, 'in stock' products will be held in store until the 'out of stock' products are available and ready to ship. We recommend that you submit a separate order for 'out of stock' products'

It works OK but id love to give the customer the option to split the cart themselves if they want to checkout using one order.

Below is an example of a store which has this option (albeit not a shopify website) but it is what i am after.

Thanks!

Ajay