Can anyone suggest how I can easily split Supplier Invoices cost into the ratio 1:5, and post to 2 separate pre-selected cost categories?

Thanks for checking into this matter here in the Community, jdsouza. I'm glad to help you enter multiple category items on your supplier invoices (bill) in QuickBooks Online (QBO).

Beforehand, if what you mean is selecting a multiple supplier on one single bill, this is not an option in QBO. We can only choose one supplier per bill.

On the other hand, we can select multiple category items on a bill. To go through with this, we can follow the steps below:

1. Open your QBO account and go to the + New menu.
2. Choose Bill and select the supplier on the Supplier dropdown.
3. Click the Category details section.
4. Add the category items to the Category column. Then, enter their designated amounts.
5. Once everything is all set, hit Save and close.

For additional reference with managing your bills, kindly check this article: Enter bills and record bill payments in QuickBooks Online.

You may also enter and manage your expenses in QBO to keep track of your business profitability.

Please know that you can always count on us if you have more queries regarding your bills in QuickBooks. We're all hands to assist you. Have a great day.