Well I suggest if your patience is running out, to open a topic on the Ubuntu PowerPC forums.
https://ubuntuforums.org/forumdisplay.php?f=328
These are people who might be able to pinpoint the problem faster than me.
I don't have experience with a dual disk setup nor when the second disk has a OS X data partition on it.
Do you still have the Apple HFS+ OS X data partition as the first partition on sdb ?
As regarding to your Open Firmware attempt,
devalias should give you an alias for your 2nd disk ,
maybe this helps understanding the drive paths in Open Firmware.
Open Firmware uses device paths like for ex. these
/ht@0,f2000000/pci@7/k2-sata-root@c/k2-sata@0/disk@0:
so for easier use there is a devalias to have a shortcut to this long path.
"https://help.ubuntu.com/lts/installation-guide/powerpc/ch05s01.html
At the prompt, type
0 > boot hd:x,yaboot
replacing x with the partition number of the HFS partition where the kernel and yaboot files were placed, followed by a Enter. On some machines, you may need to use ide0: instead of hd:. In a few more seconds you will see a yaboot prompt"
So you need
boot <devalias of your 2nd disk>:<bootstrap partition NUMBER>,yaboot