BTW what is your hardware -- is it a desktop system with integrated graphics and a PCIE card plugged in (in which case we're talking about two simultaneously active GPUs) or a hybrid graphics laptop where only one GPU is usually active at a time, or a "two GPU" laptop without hybrid graphics ?
The problem seems to be that I can only start x with the 4200 GPU. I'm not sure if I'm going the right way here, but I tried an xorg.conf generated with sudo aticonfig --initial --adapter=1 -f. (since the 6300 adapter is listed as 1) and, as I expected Xserver failed to load at startup, but this time I was dumped into a prompt and I managed to get the Xorg log of this error. These are its last lines:
[ 79.795] 0: /usr/bin/X (xorg_backtrace+0x26) [0x460566] [ 79.795] 1: /usr/bin/X (0x400000+0x64b7a) [0x464b7a] [ 79.795] 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7ffeaa2c4000+0x10060) [0x7ffeaa2d4060] [ 79.796] 3: /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/drivers/fglrx_drv.so (xdl_xs110_atiddxPxPreInit+0x87) [0x7ffea6c00247] [ 79.796] 4: /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/drivers/fglrx_drv.so (xdl_xs110_atiddxPreInit+0x205b) [0x7ffea6bd688b] [ 79.796] 5: /usr/bin/X (InitOutput+0x834) [0x474134] [ 79.796] 6: /usr/bin/X (0x400000+0x23143) [0x423143] [ 79.796] 7: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xed) [0x7ffea920030d] [ 79.796] 8: /usr/bin/X (0x400000+0x235ed) [0x4235ed] [ 79.796] Segmentation fault at address (nil) [ 79.796] Caught signal 11 (Segmentation fault). Server aborting [ 79.796] Please consult the The X.Org Foundation support at http://wiki.x.org for help. [ 79.796] Please also check the log file at "/var/log/Xorg.0.log" for additional information. [ 79.796] [ 79.805] ddxSigGiveUp: Closing log
BTW, when I was dumped into this prompt I ran sudo aticonfig --lsa and the output was the opposite of before, now the 6300 GPU is listed as 0 and as default. No idea how this works.
I hope this info is of relevance, maybe I'm just messing things up. I haven't tried Acidphase's last suggestion, I will shortly.