xubuntu – What can I do to get radeon driver working on MacPro6,1 with dual AMD FirePro D300 GPUs

I’m trying to get Ubuntu 20.04 working on an older MacPro6,1 (cylinder/trashcan) with dual AMD FirePro D300 GPUs. The only way I get any screen output is if a nomodeset my grub config, effectively disabling the GPUs, if I understand correctly. I followed the info/wiki guide for RadeonDriver, e.g.:

❯ lspci -nn | grep -E 'VGA|Display'
02:00.0 VGA compatible controller (0300): Advanced Micro Devices, Inc. (AMD/ATI) Curacao XT / Trinidad XT (Radeon R7 370 / R9 270X/370X) (1002:6810)
06:00.0 VGA compatible controller (0300): Advanced Micro Devices, Inc. (AMD/ATI) Curacao XT / Trinidad XT (Radeon R7 370 / R9 270X/370X) (1002:6810)

I also did some searches around issues w/ dual GPU setups (most of which say don’t do it, remove the 2nd GPU … I don’t think I can). Played around with adding a Device section in /usr/share/X11/xorg.conf.d/10-radeon.conf to set the BusID, e.g.:

Section "Device"
        Identifier "Radeon"
        Driver  "radeon"
        BusID   "PCI:2@0:0:0"
EndSection

It didn’t seem to do anything … I’m not sure which GPU actually has the HDMI output attached to it (or if that’s even how it works). The startup log for Xorg seemed to still indicate it was using the 06:00.0 GPU, so I also “powered off” the GPU, based on some other results I found, e.g.:

sudo sh -c 'echo 0 > /sys/bus/pci/slots/5-3/power'

This seemed to effectively disable the “2nd” GPU, but no difference when restarting display-manager (Xorg log did show it picking the 02:00.0 GPU, though). Also tried dropping to init 3 and then back to init 5 – no luck.

After all this, I’m still left with only having a display if I use nomodeset.

Any guidance? Fwiw, I don’t even seem to have non-Xorg TTYs available w/o nomodeset – or that’s not a thing anymore, maybe? It’s been awhile since I’ve tried doing anything w/ Linux desktop, but I always used to just get a TTY if I hit Alt+F1 or Ctrl+Alt+F1.