How to connect two Zoho organizations (domains) into one?

How to connect two Zoho organizations (domains) into one?

What is the easiest way to connect two Zoho organizations (domains) into one?

I have mydomain-1.com with 9 users and mydomain-2.com with 3 users. I want to stay with mydomain-1.com and have here all users moved from mydomain-2.com.