Since you are changing the tld (top level domain), simply changing $cookie_domain won't work. Browsers won't send out the cookie from one tld to another.
Ahh... that sucks... but thanks for giving me the answer. Yeah previously in such cases I had people just log into each site separately but thought there might be a nice solution.
Unfortunately the subdomain technique has other drawbacks that aren't worth it.
I don't think I'm going to bother implementing this right now, but in theory... let's say you configured Firefox to "accept third-party cookies" (false by default) could you set a cookie for both domains whenever a person logs in to one of them? Would that solve the problem?
Fletch