I've just created a new PDM using the ASE 15.7 definition, and the database definition file doesn't include an option 'EnableManyDatabases'. I'm only allowed to create a single database.
From a search of the xdb files in C:\Program Files\SAP\PowerDesigner 16\Resource Files\DBMS I can see that a few database definitions include this option, but ASE isn't one of them.