I did a separate Aatrix registration for each company with a different user name for each.  However I used the same email address for each and it seemed to work.   When I got to the point in Sage 50 where it asked me to sign in to Aatrix account, I used that companies Aatrix account to go forward.   

Hi Bonnie Ybarra , you can enroll multiple EINs under a single email address.  The specifics of 'how to' would need to come from Aatrix though.   You just have to login to your Aatrix account and enroll the other EINs.

Please click here for information on enrolling into Aatrix.

I hope this information has been helpful!

Best,

Mayla