I have a 6870 under Yosemite and I was able to patch the AMD6000 kext to get both dvi ports to work. I used the ati-personality.pl script to get the offset. The framebuffer was Gibba:
unmodified
0000000 00 04 00 00 04 03 00 00 00 01 00 00 12 04 05 01
0000010 00 04 00 00 04 03 00 00 00 01 00 00 22 05 04 02
0000020 00 08 00 00 04 02 00 00 00 01 00 00 11 02 06 04
0000030 00 02 00 00 14 02 00 00 00 01 00 00 00 00 03 05
0000040 04 00 00 00 04 02 00 00 00 01 00 00 11 02 01 03
Patched
0000000 00 04 00 00 04 03 00 00 00 01 00 00 12 04 04 01 ---- DP 1
0000010 00 04 00 00 04 03 00 00 00 01 00 00 22 05 05 02 ---- DP 2
0000020 00 08 00 00 04 02 00 00 00 01 00 00 11 02 06 04 ---- HDMI
0000030 00 02 00 00 14 02 00 00 00 01 00 00 00 00 03 05 ---- DVI 1
0000040 04 00 00 00 04 02 00 00 00 01 00 00 11 02 01 06 ---- DVI 2
Now under Sierra, I find that the ati-personality script does not work, so I found the php script mentioned earlier in this thread.
https://www.tonymacx86.com/threads/apple-intel-amd-ati-framebuffers.112299/
However, I can't match the unmodified framebuffer Gibba with the one in Sierra.
I tried putting the above data and patch in clover, and of course it did not work, that is when I started to look at the kext.