Need to keep users from two domains separate in same tenant

I have users from Domain A and Domain B syncing to the same Office 365 tentant. Domain A users are internal employees. Domain B users are B2B customers that also access some on premises data via PowerBI (which is why they are domain users and not just external invitations).

I want to prevent the users from Domain A from seeing the users in Domain B when they select anything in a people picker field (primarily) and also granting them any permissions (secondarily). The reason is because we want to prevent any accidental leakage of company information to customers. This could be something like a CapEx workflow or something else.

I found this great post on being able to set the people picker filter by AD OU via STS-ADM or Powershell in SharePoint 2013 using the UserAccountDirectoryPath property.
https://blog.josequinto.com/2015/02/12/sharepoint-2013-filter-people-picker-by-active-directory-organizational-unit/

Is this possible in SharePoint Online?

If not are there any other options?

Thanks for any help!