If you install Tribblix & OmniOS CE OpenSolaris/IllumOS Unixes on the same PC, do they need the same rpool or can they each have one? On MBR can they boot from extended/logical partitions and using LILO or GRUB2?

It's a bit fiddly, but you can have multiple distributions sharing the same ZFS pool, as different boot environments. Both Tribblix and OmniOSce have the option to install into an existing pool, and that works fine.

The trick is to ensure that the zfs pool is compatible between everything using the pool, so install the oldest distro first and then don't run zpool upgrade.

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I'd prefer to install each in their own rpool because I don't want to make a complicated rpool with more levels...

Some months ago I managed to install one or both, but didn't boot because for ZFS partition setup you have to installboot yourself. So I erased it.
        Recently I tried again but accidentally tried to create zpool on extended partition (of course, it failed) then (after zeroing that entire drive and restarting) on reboot Tribblix and OmniOS format said the NVME is an unknown drive type. The diskinfo command does say the drive type.
        I discussed more details on libera IRC #illumos and posted a bug report on illumos.org
        I managed to install both in a rpool, but now need instructions how to installboot.