Events can only be attached to a single calendar at the moment.
It would be very handy to be able to attach events to multiple calendars, so that they can be shared between many people. For example, if three guys want to meet for a reunion, we could put the event in their 3 calendars.
Now that would be handy! Feel free to fork off an implement it, the work required should be minimal.
Then again; the code for django-agenda is rather dated and does not (anymore) satisfy my own quality standard for code. Therefore it is more likely that I will rewrite most of the code before I will implement any new features on a short term.