Is it possible to write multiple files to centera under one clip id.

Yes you can.  After you create the first tag and write the blob, create another tag as a sibling or child tag and write the next blob to that

You will find this discussed in the programmers manual which comes with the SDK download

Yes, this is the design of Centera APIs:

Writing a cClip (=metadata and all their data files) to Centera is done be several API calls and can be seen as three phases:

1. Create a metadata space and add individual Metadata tags

2. Transfer ALL files, which belong to this archival compound (cClip) either concatenated as one "container" remembering the offsets as metadata or as separate  Blobs 

3. Write the Metadata space to Centera and record the returned Content Address for future reference

Each of these steps has several API calls (see step 2)