Not going to disagree that GDP is a bad measure of economic productivity, but, theoretically, in this case both the economists also got utility by enjoying themselves by paying the other to see them eat excrement. Assuming humans to be rational, it could be argued that there was a net gain of utility (if 100 $ is worth more than what you lose from eating excrement) or at least remained the same, since the buyers considered the entertainment they get to be worth at least 100 $ and that the service providers considered their service to be worth less than 100 $).
But now I feel stupid for writing this.