If your GPU is running at 90°C what temp are the VRM's are at? 110°? 120°? I hope not. Neither of those cards are getting decent airflow. The bottom card is just recycling its own heated air.
My 1080Ti gets driver resets at ~70°C but that's mainly because I'm overclocking it at 1.125V.
Maybe you could try reducing the core clocks of both cards and the core voltage.