Created attachment 848997 [details]
Case 2: No xf86 radeon driver, older 5.11.11 kernel
Same setup: no X driver for radeon, so using modesetting driver but with the
older kernel 5.11.11.xxx probably from Tumbleweed 20210403. Worked as
expected: on issuing startx GUI presented on both monitors, all seemed to work
fine.