To add another vendor partner to an individual choice order, simply navigate to the order and click 'Add Another Vendor'. This will open the search page where you can select which vendor partner to add - please note, as you are adding a vendor partner to an existing order, you won't be able to change the delivery date and window here.
Once you've selected your menu items, click 'Create Menu'. You will be shown a pop-up to inform you that the order for this additional vendor partner will use the same delivery and payment details as the existing order.
If you wish to add more than one vendor to an order, simply repeat these steps (you can only add one vendor partner at a time to an order).