Is it possible to have multiple filevault2-activated partitions on one disk?
It looks like probably not, based on a few search results :(

http://macs.about.com/od/LionTipsNtricks/ss/Filevault-2-Using-Disk-Encryption-With-Os-X-Lion.htm says:


  FileVault 2 [...] only encrypts the startup drive. If you have
  additional drives or partitions, including a Windows partition created
  with Boot Camp, they will remain unencrypted.


This thread, however, is a bit encouraging: https://discussions.apple.com/thread/5644065

On page 2, someone links to https://github.com/jridgewell/unlock which might be useful. That is, you might be able to set up a pre-login step to unlock additional volumes.