They will get pretty hot. Especially with a twin frozr style cooler that does not exhaust out the rear of the case. If you have good air flow in the case it can work. The top card will probably be about 10-15c hotter. Depending on air flow.
You may be better off selling that card and getting a good single card solution. I personally don't recommend a dual gpu card but thats just me. 580 or 680 are both great choices. But its your dough. If you have the room is always better to get a full size mobo.
But this little thing is awesome so its hard to give up for the price.
I know that is better a full size mobo but we can say i wrong bought this one you think that maybe its better to sell the card get a 680 and consider in future to get a second 680 changing the mobo?
Also are There maybe another gtx 570 from another Brand who Fit better in my solution with cooling system of the gpu?