Users in a single AzureAD directory with different domain names

I was just about to go to production and upgrade my account but I’ve run into a problem I didn’t anticipate. Newer users at the company got a newer email domain than older users and older users have not been migrated yet. All my testing was done with older users and then I tried to get an intern logged into the test environment and saw that it is treated as a separate entity. Are there any options for treating these as one big team since it is the same active directory, or is my only real option to migrate all the older users to the newer domain and go from there?