Well... maybe it is doing it wrong. I guess I could switch to 'doing it right' with a single account and still have read-write access to the shared calendars. Maybe I'll try this later. Anyhow I don't seem to be the only one doing things this way (like zaheerm mentioned, with his work and personal calendars).

In general people sometimes have multiple accounts of services for a variety of reasons (like 2 skype accounts for instance) and applications should try to support that, like I think the contacts/conversations app does nicely on the n900. Of course, even without this feature this is still a useful application ;-)