SSO / SAML Authentication

Here is my scenario -
I have four independent organizations under the group company. They all use GROUP.com for SAML authentication.
At this time, CompanyA has Atlassian Cloud applications including Jira and confluence. Today, CompanyB has approached me to have SAML authentication for their own Atlassian Cloud Jira. Unfortunately, CompanyB can't configure GROUP.com for SAML since CompanyA is already using it. This is a big limitation.

I am imagining, this situation can be overcome if we can have a custom domain for all cloud instances rather than "company.atlassian.net".

Please let me know if you need any clarification -

Thanks
/Sanal