The only thing that affects if creating a second X session will succeed on this 18.04 installation is the kernel version that's loaded. On 4.15 everything works perfectly but on 5.0 second X session fails to start and brings down existing session with it because of systemd rules for SDDM.
Adding relevant Xorg.log files and journal output for systemd and sddm with cleaned timestamps for ease of comparison.
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-5.0.0-29-generic 5.0.0-29.31~18.04.1
ProcVersionSignature: Ubuntu 5.0.0-29.31~18.04.1-generic 5.0.21
Uname: Linux 5.0.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CurrentDesktop: KDE
Date: Mon Sep 30 11:44:09 2019
SourcePackage: linux-signed-hwe
UpgradeStatus: No upgrade log present (probably fresh install)
Relevant lines from journal:
sddm: Running: /usr/bin/X -nolisten tcp -auth /var/run/
sddm: Display server failed to start. Exiting
systemd[1]: sddm.service: Main process exited, code=dumped, status=6/ABRT
systemd[1]: sddm.service: Failed with result 'core-dump'.
systemd[1]: sddm.service: Triggering OnFailure= dependencies.
systemd[1]: sddm.service: Service hold-off time over, scheduling restart.
systemd[1]: sddm.service: Scheduled restart job, restart counter is at 1.
systemd[1]: Stopped Simple Desktop Display Manager.
systemd[1]: Starting Simple Desktop Display Manager...
Odd thing is that /dev/dri/card numbers are switched between two kernels for some reason. It's a hybrid Intel/AMD laptop.