Sure, as root (su -), run 'gdmsetup' (or you can go there from System > Administration > Login Window. Then look at the Users tab for details. Just uncheck the default and edit a bit.
As to running two users at once, well you can log out and log in another user and still keep them both active, if that's what you mean. Much the same as Windows XP log out/switch user.
Linux & Beer - That TOTALLY Computes!
Registered Linux User #362651
Don't use any of my solutions on working computers or near small children.