RE: SharePoint Integration : How to share same document location between two custom entities.
If your users always follow Lead to Opportunity, you can set the opportunity to use the same document location as the lead. To do this, you could write a plugin (On Create of Opportunity) to get the document location of the lead record by querying Document Location entity with the lead GUID. And then you can update the document location of the newly created opportunity to be the same.
If the users directly create an opportunity, there won't be an originating lead set on the opportunity record. So the plugin can simply skip/ignore the document location update process - leaving the original opportunity document location as is.
Hope this helps.
Thanks