How to allow / create single login for multiple wordpress installation

How to share session between multiple wordpress in single domain?
For example: A WordPress is installed at one.sample.com or sample.com/one and other is installed at two.sample.com or sample.com/two how can I allow visitors to login in both wordpress’s using single login details. My purpose of doing this is, I want to create a webiste for multiple regions and I want to allow my visitors to register in one and login in all/multiple regions. For example there are many sites like google / yahoo it allows single login for their multiple products or subsites.