Single Registration on Two WordPress websites


I have two WordPress websites hosted on the same server; however, website #1 is the main website and website #2 is the support portal i.e. a forum similar to

1) Both websites require sign-up (registrations) before being able to access the websites.

2) My question is, is it possible have a single user registration for the two websites so that if customers have registered on the main website, they can automatically login to the support portal that is also running WordPress on a separate database but using a different theme.

3) Is there any plugin or work-around that could facilitate this requirements?

Any help or tips would be greatly appreciated.