I was curious about the same thing, having to put a glossy object atop another glossy object.
I found that Zoom Blur worked extremely well for me, melding the two images perfectly.
I simply Zoom Blurred the top layer.
You'll have to fiddle with it of course, depending on how much edge (->| |<- or ->| |<-) you want to blur and you can adjust the zoom level as to not bring the center of the top layer object too far out from the surface visually.
Mark