I know this is probably a bad idea/imposible, but have to ask. Is there a way to allow a form on domain "example.com" to submit to the user registration form on "example2.com/user/register".
I mean, it has to be possible, because if you don't turn on spam filters you'll get a bazillion fake accounts from automated bots. I'd play around with posting the values from your first site's form using drupal_http_request, something like:
drupal_http_request($second_site_url, array('method' => 'POST', 'data' => $querystring_from_form_values))