When I sold pharmaceuticals, I often saw an office nurse hand a paper bag to a patient. It contained 2 Fleets and instructions on preparing oneself for a flexible sigmoidoscopy. Then , the nurse would give verbal instructions as well.
She would tell the patient to inject the Fleets back-to-back, without expelling the first one. This would clean out the rectum, sigmoid and last few inches of the descending colon. I saw this countless times. If that proceedure was safe as a flex. sig. prep., it certainly would be safe for most patients, the exception being those with renal impairment and fluid restrictions.
I believe the warnings on the Fleet box are the "Cover your ass legally", warnings. Fleet is worried that a patient with a bowel obstruction will keep putting additional Fleets in themselves when nothing comes back out. In a court of law, the Fleet lawyers can say, we warned the patient not to use more than one Fleet Enema in 24 hours and that's what they did anyway, so it's not our liability.
I've taken 2 Fleets, back-to-back and other than having a strong B.M., the only thing I noticed was loose bowels for about 24 hours.