The only way to do this would be to modify the signin logic to see if the customer id is already logged in, and if so then force a logout. There is a "Force Logout" button for admin users when viewing customer accounts in the admin section, so you could use most (if not all) of that logic to force the logout before allowing the new admin user in.
This however, could cause issues if your admin users continue to force each other out of the admin section (probably not very productive), so instead you may want to provide a message to the admin user trying to log in that the account is currently being used.
You may also want to create separate admin logins for the different users. It's not difficult, and makes it easier to see who is making what changes via the security log should someone do something they aren't supposed to.
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