as of version 5.0 the following is true:

• eppy can have only one IDD file at a time.
• if you try to set a second IDD file, it will throw an exception.

For the near future you eppy will work with only one IDD file. What this means is that:

• you cannot open idf files of different versions simultaneously.
• Once you have set an IDD file, you cannot change it

In theory this is should not be a problem, but in practice it is a real pain. The next version of eppy will have updates that will make it easier to use. Some of these potential updates are described in issue 41

eppy will work with any IDD version. It should even work with future IDD versions.