There are many ways you could accomplish this, the first couple that are most often used would be ssh (Secure Shell - so, primarily command-line interface to start), and other graphical interfacing methods like rdesktop.

You can also use a combination of ssh and rdesktop or other methods, depending on what you're trying to accomplish. As other users have mentioned, however, it would be worth describing (in some detail) what you're trying to achieve. ;)

Since you want a GUI-interface for both of the simultaneous users, I'd suggest using xrdp and the aforementioned rdesktop

sudo apt-get install xrdp

Then run rdesktop from whatever machine you're trying to connect to the server from. A good test (done entirely on the server) is doing the following:

rdesktop localhost

.. and logging in as the 'second' user. If it works, you'll simply need to try from the other machine the same way, but there's no need to install xrdp on that system (since that's the server).

Alternatively, if you just want to connect via a shell interface:

sudo apt-get install openssh-server

... and, with the very basic default configurations, you're already setup to go. Assuming your network has no port blocking or firewalls on the server-side, you should then be able to (from the other PC):

ssh <server IP address>

Or, to test on the server just to ensure it's ready (you would actually do this on the server's terminal window itself):

ssh localhost

Definitely look at ssh --help for detailed instructions on how to specify ports, users, etc. as you please.