Share one login cookie between different mapped domains sharing one WP install

I have WP single install that has some pages and these pages have seperate domains mapped to them. So in the end it works like:

primary-domain.com/page1 is mapped and available under page1-domain.com
primary-domain.com/page2 is mapped and available under page2-domain.com
…and so on

Now I’d like to make it possible that a user which has logged in to the primary-domain.com can also be logged in when viewing page1-domain.com or page2-domain.com.

I have found great advice here how to handle this on separate WP installs (as subdomains) by defining the COOKIEHASH in wp-config.php on the subdomain WP installs. But how to achive this if the WP install is the same and only domains are different?

Sorry, my coding skills are poor so I’d appreciate some friendly help 🙂 Thx.