I've been mining for 13 hours straight with 100% usage on both gpus and temps at 65C and 67C. Is it safe to run this 24/7?
That temperature is fine for chips. Worry when it gets to 80C+ (even then it'll probably be fine). Some cards idle at like 60+
Hardware will stop itself from overheating - either by drawing less power and slowing down / increasing fan speed - or turning off the computer alltogether.
Fatigue happens if the the hardware expands and contracts a lot (increase/decrease in temperature).
I imagine going between 40 - 70C wouldn't be much of a problem tbh, though I'm not an expert.