...with MorphOS 3.18 my second monitor did not work any more. So I used the Monitor driver from 3.17 instead and it worked again. I then could adjust the resolution for the secondary monitor in prefs again.
See here : https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=13557&forum=3#156127
It might change in the future but for now the modes of the secondary display are autogenerated and can't be modified.
@Trekman: It has been reported that there is some regression with PowerMac7,x machines and secondary display support on Radeon96x0 cards, so this might be what you were running into with 3.18.