You mean something like this?
Because it's not possible in game, unless you've created your own server emulation.
+ Since the original server files servers don't trace what character is online, it has to be an account based check. So that online page shows all chars on that account that might be on, one of them is on. And the server usually updates the online page instantly, unless that player disconnected abnormally, in which then takes a few mins to update.