Create an appointment in your local calendar (user2), double click and change the "calendar" drop down box to the shared calendar (user1 with full read write access), click save and I get;
msg: invalid request: Cannot create/modify an appointment/task with organizer set to user1@domain when using account user2@domain
This happens on my install but a colleague does not get the same error on his installation.
I am however able to drag the appointment to the shared calendar and it creates a copy. The same happens on tasks.