The double ghostbuster is the pinnacle of shitting achievements in which an individual manages two highly difficult elements (or ghostbusters) in one sitting.
A single ghostbuster is when one takes a dump only to discover on inspection that there is nothing in the toilet. Where is it? Straight down the U-bend? Who knows.
Another type of ghostbuster is where one takes a dump, wipes his arse and on expection finds the paper contains no evidence a shit ever took place. A clean break, so to speak.
A Double Ghostbuster, then, is the seemingly impossible combination of both occurances outlined above.
Winston: Ray, Ray, I've just done a Double Ghostbuster!!
Ray: Yeah right, whatever
Winston: It's true Ray. It was clean as a whistle!
Egon: It's statistically 1 in 14 million to pull off a double but it can be done.
Winston and Ray: F*ck off Egon