Log Shipping: two secondary DBs on the same instance - is this possible?
It is not possible.

Ref:

About Log Shipping


  SQL Server Log shipping allows you to automatically send transaction
  log backups from a primary database on a primary server instance to
  one or more secondary databases on separate secondary server
  instances.
  
  Provides a disaster-recovery solution for a single primary database
  and one or more secondary databases, each on a separate instance of
  SQL Server.
  
  The log can be shipped to multiple secondary server instances. In such
  cases, operations 2 and 3 are duplicated for each secondary server
  instance.