The Promega rep seemed to think that this had been successfully tested by customers, but said that Promega could not do in house testing because of licencing issues.
Yes, we have used the Dual Glo assay with Gaussia and Firefly luciferases a couple of time successfully. I prefer to use live cells for assays therefore, usually performed firefly first, changed the media, waited for 6-18 hours then did the Gaussia assay. Hope this information is useful.