For sure, I have observed at certain times varying degrees of interference between certain detectors.
I believe CP had a V1 and 9500i tested in chamber and it was determined that no measurable interference could be observed in any event.
I believe Mike B. too has conducted such tests (while I wouldn't necessarily expect him to publish or announce his results).
I have spoken to a number of manufacturers about this very issue and quietly and privately they have signed-off in certain placement circumstances and with certain models.
That being said, I have always gotten to intimately know the individual performance/behavior of each before trying multiple (again with careful placement). Remember I have been driving and using radar detectors for more than 30 years! Bottom Line: Some get along, some don't.
I will share one story that I actually experienced with Steve (Happya$$) in the town of Sheffield Village, Ohio. I believe we had one STi Driver and two Valentines carefully placed across our windshield.
We had a quick 34.7 IO frontal moving encounter and one of my V1s (an older analogue temp compensation circuitry model) stayed quiet. I mean it completely missed a point-blank IO shot at our front.
Later that week I visited our friends at Valentine Research and lo-and-behold, the older V1 was in need of a tune-up. To their credit, they didn't blast our setup. They stepped-up and simply tuned the V1 back into its top form. Had I not had another detector running at the same time, I simply wouldn't have known that one of my V1s was in need of service. I have always admired the engineering integrity of Valentine Research and its carry-through to their customer service.
Of course, today, both the STi-R and the 9500ci serve as a mutual check-and-balance system. Granted, it ain't a cheap setup, but then again, at the speeds I travel, neither would be the tickets that I would otherwise be subjected to had I not had the privilege of their ownership.