Hey Peter, sorry for the late reply! Normally I'm faster than this.
Your question is a good one. The current situation with shared folders and Espionage is that access to an encrypted folder's contents can be shared, but control of the folder cannot.
Basically, you'll be able to encrypt encrypt a folder outside of your home directory (say, somewhere in /Users/Shared) and set it to have a "Public mountpoint" (using the checkbox in Espionage). Then, from your user account, you can unlock the folder and then use OS X's Fast User Switching to switch over into your wife's account. From there you'll be able to access the contents of the folder.
At some point in the future we'd like to expand Espionage's sharing capabilities so that folders can be manipulated more easily from multiple user accounts, but at the moment each folder can only be controlled by a single user.
Hope that answers your question, let me know if you need any clarification!
Kind regards,
Greg, Tao Effect