Celestron sell a piggy-back mount for their 5" to 14" SCTs. It's intended for mounting a camera (for wide sky shots), but it will take a modest spotting scope as well.
http://www.celestron.com/browse-shop...iggyback-mount
Note that its made of plastic, not metal - it seems perfectly sound, but an 80 mm spotting scope piggy-backed onto an SCT might test the limits of the adapter and the tracking mount. (Although the location of the threaded adapter under a spotting scope is often much closer to its balance point than a DSLR with a protruding telephoto lens.)