Org structure is very strict when it comes to siebel.
Now In order to make data visible from other orgs, partner to be setup, they need to be created as an Organization or underneath an existing Partner Organization and that Organization needs to be added to Siebel objects in order for the partner to see those objects ex Accounts, Opportunities, Price Lists, Products etc.
Here just adding org under each other will not help but you need to add Org to the objects that Org needs to see.
This is standard PRM practice and enforces security so that partners see only what they are allowed to see.
Although Siebel provides the ability to add an child organization below an organization – there is no place in Siebel that flattens out this org structure for providing access.
Just my thought..its open for further discussion.
There might not be any documentation for it.
But considering how Siebel is strict when it comes to Organizational structure, this needs to be handled with outmost care.
I would suggest to create an SR with Oracle support and give reference of this thread for better clarification. It might happen that we might need to contact internal product experts as well for various approaches.
I hope it helps.