Circling back to confirm that it is now possible to belong to multiple Cloud organizations—i.e. organization1.retool.com and organization2.retool.com — with a single email. 
Thanks for expressing interest in this functionality!
Some things to keep in mind:
If you use a single email to access multiple Cloud environments, you’ll no longer be able to login directly from login.retool.com/auth/login. Instead you’ll need to access the organization-specific login page for the organization you wish to access. For example, if you want to login to organization1.retool.com you’ll need to visit organization1.retool.com/auth/login.
If you attempt to login from login.retool.com/auth/login, you’ll receive an email with links to the organization specific login URLs for all of your Cloud organizations to guide you to the correct page.
Go forth and belong to multiple Cloud orgs if you'd like! 