Reducing noise in Cycles renders by compositing two low sample renders?

I swear I saw a short tutorial on how to reduce noise here some time ago but I simply can’t find it anymore. You rendered two or more 1000 sample renders in the OpenEXR format then combined them to create the final image which had practically no noise at all. Basically getting a final image that is equivalent to some 10,000 samples by rendering a couple of 1000 sample images. I could really use that tutorial right about now.