How do I have 2 management accounts

You cannot have two users if you expect to have a so-called root user [1].

However, it is possible to pass close privileges by creating an IAM user and giving it an AdministratorAccess policy and a Billing policy.

See "Tasks that require root user credentials" [2] for the difference between a root user and a root user. These are tasks that can only be performed by the root user.

[1] : https://docs.aws.amazon.com/IAM/latest/UserGuide/id_root-user.html

[2] : https://docs.aws.amazon.com/IAM/latest/UserGuide/root-user-tasks.html