2 sites – 1 user database

Hi

I run 2 sites (2 different domains) with wordpress.

Both hosted separately, but by the same host (Go Daddy).

I want people to be able to sign in (comments and also hidden content – via PMPro – or similar), and have the same login details for both sites – but only needing to register once.

I’m not concerned about keeping people logged in as they switch from site to site, but holding username and password for both is really important to me.

How would I go about this?