Login twice on same account? Good or potential security risk?

So I logged in today to find that 2 of my guild leader's characters, on the same account, are logged in. Of course he did it but it seems now that an account can be logged in twice either using 2 separate computers or 2 different user accounts on the same computer.

Let's say ID: MS user1 is logged in using character X, that same ID can be logged in on a different computer using character Y.

Any thoughts?