I responded to a PM from this user, and mentioned that yes, you certainly can do so. I know if you click the Enter Draft link from your team page, it definitely uses a different window name and should work just fine. If it isn't working for mock drafts, I'll talk to the guy who worked on that. There is no reason it can't work, they're probably just opening up both drafts in the same named window or something. For independent reasons, you can't sign up for more than one mock draft that haven't started yet, but that is a different matter.