I heard that it is possible to be using the X windows while being logged in as two or more users...
But I also heard that in order to do so, you have to edit a system file.

Which is the file I must edit and what modifications should I do?

I can't think what configuration you would need to edit.

You can use the same X session by suing to another user and starting programs or by starting another X session (where 1 is the number of the next desktop - 0 is the first):
X :1

You can then start programs on that display. To allow access to a display by other users/hosts you may need to change the access control with "xhost" and to choose a display you cna change your users dispaly variable - ie:
export DISPLAY="127.0.0.1:0"
or for the second desktop:
export DISPLAY="127.0.0.1:1"

you can also use a really simple program that is included with gnome called xnest.