Limit number of concurrent users switching to root account?

This is for Ubuntu 14.04 and Centos 7.

I need to limit the number of users actively running as root. i.e. Logged in as root on the CLI.

Basically, I want only one user at a time to be able to run commands as root. The purpose here is auditing.

I looked into setting limits in /etc/security/limits.conf but the pam_limits.so module only seems to affect logins. Or login shells. Not sure. But whatever the specifics, it does prevent a user from SSHing to a box more than once, but does not prevent more than one user becoming root via "sudo su". Thus setting limits.conf can still allow more than one user being logged in as root at a time.

Here is the limits.conf line I tried that limits this:

root            hard    maxlogins            1

Next I tried limiting users in the @admins group. I figured that those users are the only ones allowed to sudo su to root (based on custom sudo rules we have in place).

@admins            hard    maxlogins            1

This seems to do what I want but seems clunky/wrong. Call it a gut feeling -- I don't quite have a handle on what I see as wrong about this one.

Finally, "Why?". Why do I have this requirement?

We are trying to implement controls to meet PCI-DSS 3.1 requirement 8.5 "Do not use group, shared, or generic IDs, passwords, or other authentication methods" -- emphasis on the "shared". In a Windows environment, you just grant users authorization to do whatever, and no one shares the primary Administrator account. Linux environments are designed such that for some things, you really want to be logged in as root. There must be a PCI-compliant way to solve this problem in a Linux environment.