Two GUI applications as different users on the same screen

Run the following command to give userBname access to the display:xhost +SI:localuser:userBname You may use xhost + to give access to everybody; however this is insecure. Better is to give access on a per-user basis. The correct syntax for xhosts entries for local users is SI:localuser:username.

Then su userBname, export DISPLAY=:0.0, and finally run your GUI program. Although you granted access to the display earlier with xhost, you must also set the environmental variable DISPLAY so that programs know where to place their graphical output. Allowing different DISPLAY variables for different users allows multiple users connected to a single machine to use different physical terminals.