aonor :
PyjamasCat :
Is there decent spacing and good airflow between them? My single GTX 670 idles at ~35°C
There's about 5 CM gap between them.
That's good. I'd say, it is likely that the cards are dumping heat back into the case and the hot air is only removed at a certain rate, but 47°C for long periods shouldn't do any damage. What are the temps like under full load? That is usually where damage occurs.
General info on case cooling and fans:
Ensure the hot air is moved out of the case. Case fans should be arranged so that the air all moves in the same direction, preferably up and out the back of the case but there are other ways to configure fans. The case should be in a cool place with plenty of room to breathe in fresh dry air. Preventing dust with filters and/or positive case pressure can help in the long run, but dust filters may slow the airflow. Sometimes fans can hinder rather than help, such as a side fan blasting the GPU, and actually stopping it from receiving fresh air. It may be better to have it on a slower speed if possible. Cables can also get in the way, so if you can, tie them back or run them around the fans air channel. Some cases have space behind the other case door for cables to be run, but cable length and actual space may stop you from using it.