You would be both using the same username and password, potentially letting the other person change the password and gain control over the site. You can not set up a separate login for cPanel.
Sure,
You are talking about them typing their domain name into a browser and it showing your home page right? Point your DNS to NS1 and NS2.TOTALCHOICEHOSTING.COM and open a ticket asking them to park that domain on yours.
This would be difficult to implement across some 70 servers as well as new ones as they come aboard. We would also have to know if all of our clients are using their domains SMTP servers or those of their ISP. This doesn't count resellers that would have to get the information from their clients and pass that information on to us.
I can not see this being implemented at this time.