Single Signon across domains - no subdomains

I need to create a single signon for domain1.com and domain2.com. I found only solutions for subdomains or multisites in this forum, but that isn't what I need.

I've found out, that a single signon would be the best for my needs:

Requirements

• no subdomains
• no multisite
• both domains are wordpress sites
• domains are on different server/webspace/database
• We have FTP access to both domains
• The registration form on domain1.com should not be changed if possible
• User data should send to domain2.com and the user should automatically logged in, after clicking a link on domain1.com

I read also this thread: cross domain login But it seems really comlicated.

Question

How to create a single signon for this requirements? Are there Plugins or some framework to use?