At 91c, you'd better turn something down or you'll have a fried GPU. Run GPUZ and post what it shows.

Set them both the same. Also, when I see usage lines like this, it needs a bit more Mem speed. 84c, lotta heat. If this doesn't work, I would lower the Core clocks. Are you doing anything else with this box or mining only?

Some GPU's just won't work properly with mining, maybe it's nearly done or something else. I have 2-6950's and 2-5830's that won't overclock worth spit but run fine at stock speeds, minus Mem speed.

Intensity on one is 9 the other is 12. I'll wager GPU0 is crashing.

Wheels on the press go round and round..........

Unlock the over-volting, restart afterburner and up the voltage in small increments. Watch the temps. Also, drop the Mem speeds by a lot, say to 800-900 Mhz.

File a dispute and see what happens. If it was "gifted"...well......

That's for 1 card, is the other not listed, it doesn't look like it? You can also install Afterburner or use GPUZ to check.

Looks like one GPU has a lower clock. Look at CCC and see for sure.

Need dummy plugs?

That store has all kinds of fans. Mine were Diamond cards and they worked fine, although a little shorter than the originals height. Measure your fans diameter, height and bolt pattern. Also measure the pigtail length. I've bought replacements for both AMD and Nvidia and all have worked just fine, slow boat from China but they were packed well and all worked as intended.

The 5770's that I have had were stock cooling and running Boinc wide open and overclocked yet they stayed under 70c. (alt fan profile was used)
I believe they will withstand 100-104c before disaster strikes, just not sure how long one would last at that level. Cramped case or poor cooling perhaps? Lead free solder is crap IMO.

IIRC the 6770 is basically a rebadged 5770 with some minor updates. They normally run quite cool.

When the GPU is seen as idle in the OS, it underclocks. I usually see half the core and memory speeds.

I've bought several fans from this place for both AMD and Nvidia. Replaced the fans in my 5830's with these as well. They cost more than $10 though.
http://www.ebay.com/itm/180885945677?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
If you order, measure everything beforehand.

Only a dual setup? Are you planning to crossfire?

LMAO Excellent! Bitcoins, where art thou?

http://www.guru3d.com/articles_pages/his_radeon_6770_iceqx_turbo_crossfire_review,1.html  Crossfire for mining, I wouldn't. If you don't crossfire you will still need a dummy plug.

You will need a dummy plug and welcome to the world of mixed GPUs. You can make one pretty cheap. I would put it on the Nvidia and the monitor on the AMD and see if mining works.
http://www.overclock.net/t/384733/the-30-second-dummy-plug