Well, I tried this on two sites powered by Seditio, but I believe Cotonti has no difference here.
I think its important to mention some other problems you may encounter:
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The $usr['auth'] keeps the cached auth settings for current user. Having two sites with different "rights" means that you'll need two separate auth fields in sed_users table. (plugin required)
Cookies cant be shared across different domains. All your sites must share the same "top level" domain in order to keep the user "logged in" between each site.
the sed_users table is updated each request, so this may produce a little overhead in database
Perl - the only language that looks the same before and after RSA encryption.
