You can solve it but this will require a small change in the Python code which may lead to update conflicts in the future. Hence I would not recommend it.
Can we include items from multiple sales orders into a single delivery note ?
This can be done for items from multiple sales orders.
For the Customer PO field in Delivery Note Item, mention this in
Options Fetch From of that field, assuming
against_sales_order is the Sales Order Link in that Item row :
against_sales_order.po_no You can keep this field read only.
Now create a new Delivery Note and Get Items from multiple Sales Orders for that customer.
Now check if the Customer POs have been mapped to the Delivery Note Items or not.
Why I think this should work
Every item when fetched from a Sales Invoice/Order will contain a backlink to it. Now it the Customer purchase order number is mentioned in the correct field of Order/Invoice and you too have referenced the correct variable as above then it will get mapped automatically.
Making Deliveries from both Sales Invoice and Sales orders
In case you make Delivery Notes from Sales Invoice too, then you can create a similar field but with different Fetch From value i.e.
against_sales_invoice.po_no In this way, it becomes flexible for you.
For multiple sales orders you can make a single delivery, but if you are make it from a a Sales Invoice then you cannot do it for multiple invoices unless you do it manually for each item - a practice which will be restricted depending on the number of items. I am not sure about how okay is this practice as far as accounting standards go.