The case is open, airflow is fine, room temperature is low.
Isn't it weird that one of the cards stays cool, and the other gets hot?
Without some fans, a case with the side panel removed would imply that the airflow is nowhere near fine.
You need far more than just convection to move the air around.
Hate to break your heart but cheap PC cases suck dicks when mining comes into play.
At the very least you should point two 120mm fans at those cards, e.g. one on the side panel (the more fannage the side panel can house the better) and one blowing from the front panel or from the bottom. Also, you need as much exhaust fan-power to make sure the hot air can be removed from the case double-quick.