Combine users of two different wordpress sites

Hi
I need to combine users of two wp sites (they have different domains) so that if someone signs up for one of them s/he will be signed up automatically for the other one, and also if s/he logs in to one of them he will be logged in to the other one too. ( a good example is google services, when you log in to gmail.com you are automatically logged in to youtube.com).
Any help is appreciated…