Shared calendar

[Alvin]

At the moment, I'm sharing my calendar with myself, using Akonadi. The  
Calendar is an .ics file, residing on an sftp server. My office pc and  
home pc both use Akonadi to edit the .ics file.

So far, this has worked well. Sometimes, the calendars do not seem to be  
in sync, but lately this system seems to be more stable.

Now, I do not trust this to edit the calendar from both computers at the  
same time. After all, Akonadi is made for single users. So, how do I  
extend this? I want my wife to be able to see and edit the agenda. What  
would be the best way to work together? (She uses Evolution, but that can  
be changed.)
Do I need a Caldav server, do I need Kolab, eGroupware? What's the  
(K)Ubuntu preferred way to do this? Should she just have another calendar  
and mail me the appointments? Do I need to look at Google calendar?