That would be Mozilla Sunbird as I mentioned. It is a desktop application just like iCalendar Lite. Or, you could still use iCalendar Lite. In either case, move the existing calendar file to a user neutral space, like the root of your C drive, or create a new one there. Then you set each user's settings to use that location and file for the calendar. You may also need to grant read/write permissions to both users. I am not sure how iCalendar Lite will handle this, but with Sunbird it is possible.