I would like to go back to a question I asked before and this time I

Do not know if this can help, but I also need to invoice a group of customer in once (A company group for its subsidiaries, where Company group and up to 5 of its subsidiaries are customers). What seems to be more easy in my case is that the group is always paying for its subsidiaries.

So I created an Account db, that I only use for financial movements. All 4 subsidiaries and the Company group shares the same account, and my invoice is made on the account, not the Customer Id.

My endmonth invoice summarizes all the purchases made by the group and its subsidiaries.

So why not group your customers, either in a Account db, or populate the line item with a Lineitem id (that would be common to the number of customers set).

Hope this can help.

Im not sure exactly what the problem is, but could you solve it by having an additional field in orders (lets call it t_linkID). this field would be set automatically to the order t_uniqueID when a new order is created. When a customer wants to travel with someone else, you would have an "additional order" button that would create a new order and set t_linkID to the previous orders t_linkID (and, of course, set a new t_uniqueID for the second record). Individual orders would show line items from t_uniqueID relationship and invoice would show line items from t_linkID relationship. Clicking on the invoice buttom would just take you to related record t_linkID. It wouldnt matter who is paying, both would be related to the invoice.