One of my patient in the hospital just asked me this same question few days back and i found it rather interesting . The Human body have a lot of muscles (both voluntary and involuntary) that are involved in helping one to both urinate and pass stool. These muscles have dual functions — holding urine and stool in, and passing urine and stool to the outside world. The muscles that help to hold in our stool are way stronger and larger than the muscles that hold in our urine.
The primary muscles that function to hold in stool are the internal and external ANAL SPHINCTER. The internal sphincter a totally involuntary organ, and as poop passes through this sphincter, the muscle slowly relaxes. The poop then contacts the external sphincter, which is a totally voluntary organ. At this point, we feel that there is an increased pressure in our rectum, and we likely feel the need to defecate. If a toilet is close by, we sit down and willingly relax our external anal sphincter, and the poop passes through. If, however, we would rather not have a bowel movement at that moment, we voluntarily tighten our external anal sphincter, and the poop is retained in the rectum.

Control of urine is similar. The muscles are smaller and less stronger, but the actions are the same. There is both an involuntary internal urethral sphincter and a voluntary external urethral sphincter. The internal urethral sphincter opens when pressures inside the urinary bladder are great enough; we can hold back the urine by tightening our external urethral sphincter, or we can urinate by relaxing this muscle.

Then we ask ourselves why do we urinate when we poop?when the answer is quite clear you do so because the pelvic floor muscle relaxes when we poop, in addition to allowing stool to pass, it decreases the tension in our urinary sphincters, thereby allowing urine to flow.

It is easier to have control over your anal sphincters than your bowel,so experiencing this it means YOU ARE PERFECTLY NORMAL. Also, the control of urine and stool differ in men and women because the anatomy of the male and female pelvis are so different.

I Hope you find this helpful, and Please I need your upvotes so it can reach more people who are scared something is wrong with them.

MSN,

Natalya Yvette.