Alex, I've tried 3.15-rc2 and your new ucode files from
http://people.freedesktop.org/~agd5f/radeon_ucode/ for both cards now, but
still no luck with xrandr and both cards. Firmware is correctly loaded. Same
crash.

[   21.174824] [drm] initializing kernel modesetting (HAWAII 0x1002:0x67B0
0x174B:0xE285).
[   21.436057] [drm] Loading HAWAII Microcode
[   21.442757] [drm] radeon/HAWAII_mc2.bin: 32364 bytes
[   21.698816] [drm] initializing kernel modesetting (PITCAIRN 0x1002:0x6818
0x1682:0x3251).
[   21.699439] [drm] Loading PITCAIRN Microcode
[   21.701027] [drm] radeon/PITCAIRN_mc2.bin: 31100 bytes