Can passing faeces and urinating occur at the same moment and if not, why?
Now I don't have any medical knowledge really or background but the only conclusion I can come to is that faeces press against the connection internally from the bladder to the urethra preventing urine being released until the pressure is removed from the connection.
There are voluntary and involuntary muscles on urination. The default circuit is to close the involuntary during and before bowel. It is a way of the body forcing you to clear the bowel.
They certainly can occur at the same moment, but there are also situations where they might not. Similar to your hypothesis, if someone is for example severely constipated they may not be able to urinate until they have had a bowel movement due to the urethra (the tube that allows urine to leave your bladder) being pinched off from the pressure of the stool.
