1) What PSU power do I need in order to run both graphics cards whitout stability issues?
Your 850w is more than enough to power up the 2 ti's, ensure that it is a decent PSU brand like Corsair,EVGA,Seasonic
2) With the PSU that I have (850W), using msi afterburner to reduce the max power of each graphics card, is it possible to avoid this stability problem? If so, to what percentage should I reduce?
It must be your overclocking issues, try these clocks
Power limit 70%
Mem Clock -100
Core Clock + 100
this is also a good read for 1080ti clocks
https://bitcointalk.org/index.php?topic=1828831.0goodluck