Always fun,I had a lovely toaster running in this type of config with Crossfired R9 280x's
On to the topic at hand,you'd have to look at the Case in use as well as the cards,and work out a good Airflow setup to help keep them cool
Old case I used a combo of Antec Spot-cool (
https://www.amazon.com/Antec-Spot-Cool-SpotCool-System/dp/B000I5KSNQ ),regular case fans ( Airflow models ) and the Case's inherent characteristics to route hot air out of the machine quickly
Biggest issue tends to be the self-feeding beast that is hot-air rising to each card and feeding the next. Creating a positive-pressure setup where top is set for extraction and the Side-panel, front and bottom to pull some juicy cold air in helped