You can use this option to split a cost between more than one Purchase Order.

This can be done from the View Purchases screen, which can be accessed by going to Purchases | Purchase Orders | View Purchases.
Choose one or more orders by selecting them using the first column, and then hover your mouse over the cog icon column header to open up and use the bulk action cog. You will then be able to select the 'Add Shared Cost' option:

A pop-up window will be shown where you will inform all the details relevant to that cost, as you would do with individual costs. The system will populate a grid with the orders that are going to be affected so that you can review your changes before submitting the cost.

A helpful comment will be pre-populated in every cost mentioning all affected orders to assist you in the reconciliation process, but that can be overridden. All other fields will behave the same way as the individual costs do, as shown in the screenshot below:

When you are done, press the Add button to have your shared cost added to the selected orders.

The cost you entered in the pop-up will then be split proportionally among all the selected orders based on their weight towards the total. 

For example: when splitting a cost of NZD 100 among 3 Purchase Orders, the system will take into account how much of the total each purchase order represents before deciding how much of that cost to attribute to that order. If the sum of the cost of those 3 Purchase Orders is NZD 1000 and one of the orders cost NZD 730, then it will attract 73% of that shared cost.

All the selected orders will now have a cost line that refers to that shared cost where you will be able to see all orders that share that cost and, if necessary, manually amend the cost distribution.

Note: The existing Landed Cost of the products in the Purchase Order must have a value greater than 0.01 in order for the Shared Cost to apply successfully.