SQL Server database snapshots for integration testing
Unfortunately, it's by design. 

Taken from BOL page "Revert a Database to a Database Snapshot":


  Limitations and Restrictions 
  
  Reverting is unsupported under the following conditions: 
  
  
  The database must currently have only one database snapshot, to which    you plan to revert.
  Any read-only or compressed filegroups exist in the database.
  Any files are now offline but were online when the snapshot was    created
  


As an alternative, you could drop the first snapshot (db)_Basis. I can understand that this seems very limiting but look at it this way: snapshots are sparse files based on the original data files, so reverting to a specific snapshot would invalidate all snapshots anyway (the base data files would be changed by the revert operation). The limitation can be annoying, but doesn't look unreasonable.