How to enable xhost access from second user when display:0 is on first user?

Here is my situation: I have two (sudo) users on a machine: 1. userA (created first on the machine. This is also where display :0 is attached) 2. userB (created later) Being on userA I do:

export DISPLAY=:0.0
xclock

And the clock opens on DISPLAY:0 as intended. Now, I want the same steps to work on userB. But unfortunately, it shows:

No protocol specified
xhost:  unable to open display ":0.0"

What I tried: 1. Creating a trusted xauth key using: xauth generate :0.0 . trusted which again shows unable to open display ":0.0" 2. Copying the /home/userA/.Xauthority to /home/userB/.Xauthority. (Note: I did not see any key pertaining to display :0 in the userA xauth list, even though it works.) 3. Creating trusted xauth key for :0 on userA and copying that key to userB. None of these worked. **What worked:** 1. I log-in to userA. I export DISPLAY=:0.0 and then xhost + to enable access from all clients. 2. I log back in to userB. I export DISPLAY=:0.0 and then xclock. I want to eliminate Step 1. I do NOT want to log-in to userA each time. And I did try to xhost + from userB, which shows error already shown above. How can I run xhost + from userB, while also making it think it has the DISPLAY:0 access? **Edit**: As mentioned by @user414777, I was able to:

# On userA
$ xhost +si:localuser:userB

# On userB
$ export DISPLAY=:0.0 ; xclock

And make it work. But these changes don't stay post-reboot.

# After rebooting, on userB
$ export DISPLAY=:0.0 ; xclock
Invalid MIT-MAGIC-COOKIE-1 keyError: Can't open display: :0.0

I'm running Ubuntu 18.04 LTS, 4.18.0-15-generic **SOLVED** I eventually went with a slightly insecure and modified suggestion by @user414777. Instead of adding the change in ~/.xsession or Xsession.d, I added that line in /etc/profile, which applies the change for all users.