If your 1st adapter is exactly on top of the other (pcie slots too near), then no matter the custom fan speed you use, it will always overheat because it sucks hot air directly from the second adapter during load.
If there is enough space between the two cards inside your pc case, and you have a good internal case airflow, then 90°+ C is too hot for the 1st and 80°+ C is too hot for the 2nd as well.
Leave automatic fan for casual pc use only.
Use msi afterburner to make a custom fan speed of 50-70% rpm, before launching a demanding/ stressful game, so as to maintain a temperature mark of up to ~78° C. Adjust fan speed accordingly and/ or case airflow.
gigabyte X58A-UD3R rev.2, core i7 975 3.9ghz (stock voltage), windows 8.1 64-bit, corsair 12gb DDR3 1600mhz 7-7-7-20 RAM, xfx HD 5970 + gigabyte HD 5970 (950mhz/ 1100mhz), coolermaster ultimate 900w PSU.
amd catalyst 14.12 Omega Driver