I am used to being able to launch a second X session in TTY8 with exec startx but instead I'm getting an error:

X: user not authorized to run the X server, aborting.

And then in TTY 8 I end up with a stuck cursor and no login at all. Just the blinking cursor in the very top left. How to I kick up a second x server so that I can log in as another user there?

Update

With help in the comments, I've gotten somewhat farther. From the terminal in an existing xsession, I can do exec sudo startx to get a new x session in TTY8, logged in as that user. If I hop to TTY4, log in and do exec startx I can get a session, too.

Here's what still isn't working: the session I get is a a working x session but without any launcher, top bar or Unity window management. So ctrlaltt gives me a terminal and I can launch applications from there, but I can't switch between them and if I minimize a window there's no way to get it back.