Hi
I had the same issue as well and in order for me to allow to logins
into different instances i created an alias to the email so if you
have Jdoe@... i created an alias johndoe@... and everything came to
his official inbox and was able to login to the second production with
the second email address.
So the only thing was that they have to login with the alias name.
Please let me know if this is what you were referring to.