In all UNIX/Linux systems (that I know) only one user can be logged in at a time on each "terminal" (the console being a terminal, if you connect in via the net, that's a different terminal). So, what you are asking to do doesn't make sense in a UNIX/Linux context. However, once you are logged on there are many system facilities for "acting as" the other user. Look at the commands
su, for one approach. It may also be that your goal can be achieved by the correct use of UNIX permissions.
Since your question didn't say why you wanted to do this thing, which sounds silly to my ears after 30+ tears of UNIXing, I can't actually tell you how to achieve that goal. I can only tell you that the approach you are asking about isn't going to lead anywhere useful.