If you have a window manager(wm) running (KDE in your case) you have a few options:
- Start another wm session after becomming the other user in a terminal (not my choice)
- Logout from current wm to commandline prompt. I do think that your box starts with KDE (no commandline first and manually start the wm), so you will be presented some window for login in, and not go to the comman line.
This leaves the following:
You can go to a different 'terminal' while running a wm.
With the following key-combo you can goto a different terminal:
The F stands for the function key, 1 to 6 are the 6 terminals that are present (depends a little on your settings).
So ctrl-alt-F2 would give you a new terminal, your current user running the kde session is still active and you can return to it by alt-F7
You can do anything you want in the new terminal. Running commands, starting a wm, killing a frozen wm etc, etc.
If your box has the power you could run 6 kde sessions, belonging to different users. All at the same time.
If things are still not clear: Keep asking