I want to show both the TIN Numbers mine as well as Customer’s on Sales Invoice
Yours you can setup letterheads
and for customer’s TIN no, you have to add custom feild. Check https://frappe.io/kb/customization/fetch-custom-field-value-from-master-to-all-related-transactions
That can only be seen on forms, I want the same to be shown on Invoicing Printing Report.