Same login for 2 blog sites

Hi there. I have 2 sites with separate domains and they need to stay that way, (no way to make one a sub-domain of the other).

I need to connect them in some way so they use the same WP admin login. I looked into wp-hive and multisite, but from what I have read, one site has to be a sub-domain of the other.

Does anyone know of a way to do this without making one site a sub?