Create a small WP plugin for controlling concurrent plugins

I'm interested in a small Wordpress plugin that allows me to set a limit of concurrent logins for each user. The default should be 1 login max, otherwise the previous or first login session is destroyed. I should be able to set an amount of logins for certain users (like 6 maximum logins for userx). I've seen similar plugins under 50 lines but they are not exactly what I need.