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The idea is to have on a Ubuntu machine 2 X screens. One for the local ubuntu and one for a remote machine, in the example a Raspberry Pi.
user@ubuntumachine> X :1 -query raspberrypi.local
Then because it doesn't work:
email@example.com> export DISPLAY=ubuntumachine.local:1 firstname.lastname@example.org> lxsession
If you want to export the variable at startup, add the same line in /etc/profile.