i have used two doublers on my 10x42 slc's, and it works just fine, but the field of view is cut drastically. it also worked on my buddy's 15x50's. it is a good idea if you are really trying to pick apart a hillside, and fov is not that important.
i also have gold ring 12-40x60 spotter, and i think it is as good as my buddy's swaro. The biggest thing is to get a stable tri pod......