I just read that strongbox has a unique method of doing this with their software running to secure both sites:
If you have more than one Strongbox protected site you can use the Strongbox handoff function to link the members area of one site to the members area of another in a cryptographically secure fashion.
Ray (the owner of strongbox) is a great guy and I would highly recommend strongbox to protect your members areas from hackers and password traders anyway!

cheers,
Luke