“Multiseat” occasionally gets confused with other things. Here, I’m talking about multiple people simultaneously using a single computer, each with their own directly-attached monitor, keyboard, mouse, [usually] graphics card, audio and graphical session, etc. Each “seat” acts like a standalone computer.
And, well, it’s broken. Somewhere between 22.04 and 23.10 it went wonky. Sometimes there’s no greeter, sometimes there’s a greeter but then no desktop after login for secondary seats. This is probably down to the heavier introduction of Wayland and all the things that surround that, but at the moment a stock Ubuntu install won’t do multiseat. You have to use another DM like SDDM or LightDM but they come with their own piles of integration bugs.
Relatively few people use multiseat. That’s obvious based on the number of people complaining about it being broken, especially on here. But for some people, it’s a critical system. I’m trying to raise awareness of this before we get to the next LTS so that this isn’t a problem for LTS-track upgraders.
What’s not entirely clear to me is if this is just GDM or GDM+UDEV or some other frilly combination of systems but simply running a botched together compile of GDM 46.alpha does not work, though that’s what’s running on Fedora Rawhide, and that works. That’s why I’m here, not Launchpad. I need direction.
I’m floundering in an upstream bug report and a helpful contributor showed me you can actually test this stuff quite easily with qemu. udev knows how to handle pci-bridge-seat:
qemu-system-x86_64 -cdrom noble-desktop-amd64.iso \
-smp 2 -boot d -m 2047 -vga virtio -display gtk -enable-kvm \
-device pci-bridge-seat,chassis_nr=2,id=head.2 \
-device virtio-gpu-pci,bus=head.2,addr=02.0,id=video.2 \
-device virtio-keyboard-pci,bus=head.2,display=video.2 \
-device virtio-tablet-pci,bus=head.2,display=video.2 \
-serial mon:stdio
You get two screens. On Fedora, that second screen is kinda functional (I think the live environment is what’s blocking a proper desktop). Ubuntu’s second screen is completely dead. 
I don’t know what to do next. That feeling —and that of not having a bunch of other people here to complain with— has seen me write-and-not-post a few bug posts since I’ve had this problem. If you’ve any idea where this needs to go to get the correct attention, please poke me on. If you’ve got any ideas or experience with these subsystems, I’d love to hear from you.