As promised I checked too, using xorg and radeon drivers currently from sid (the version of the server packages is the same of the packages in testing, while the driver's version is not --used 6.13.0-2--), kms enabled (it's the default for radeon), and can confirm it works without problems here too.
I also tried to switch the cards "on-the-fly" (= with Xorg running) echoing "DIS" and "IGD" instead of the "delayed" equivalents DDIS and DIGD (that perform the switch when shutting down Xorg) to check if using them could lead to breakage: no problems in this case too, as the echoed option is ignored System is an Asus M51Ta, with ati+ati hybrid graphics. D. -- Grabel's Law: 2 is not equal to 3 -- not even for large values of 2.