Am doctor, you are essentially asking about a fistula (abnormal communication) that would allow urine and bowel to mix. This would allow for communication of gut bacteria to then invade the urinary system which would then easily cause a kidney infection which would then seed the blood resulting in bacteremia. Bacteremia is bad, like you die from infection unless you are on IV antibiotics and get the source under control bad. As an example, colovesicular fistula is a communication between the colon and bladder which often results in bad infections and needs to be surgically repaired. Hope this helps.

Consider, for a second, what would happen if and when the pressure in the colon exceeded the pressure in the bladder, and a urethra connected the two: fluid in the colon would backfeed into the urinary tract. The kidneys would ultimately be exposed to intestinal flora.

The prevalence of UTIs even with the "air gap" of separate waste streams tells me that this would be an extraordinarily bad idea, at least with our current biology.

If we had evolved in a more arid environment, there might be some advantage, but if we had, we would have a significantly different system to maintain homeostasis: it would have to be significantly more resilient to bacterial contamination than our current system.

I have Crohn's disease and therefore already pee out of my a$$ on a regular basis.

0/10 would not recommend.

(obv. not exactly what OP is asking but i imagine it feels the same)

Then you'll basically have a cloaca, and perhaps you'll ended up like bird and reptile which can't seem to hold their shit and also have watery shit. One of the great things about mammals is they're able to hold their (relatively) dry shit until they found a safe place to dump it.

The kidneys already recover water and salt. Google the loop of Henley.

What we pee is what our body considers in excess. We intake a lot of our water from food, so we need a way to expel that. Peeing is much nicer than bloating and constant Diarra.

Well, then urine trouble because that would be like having liquid poo all the time. Gross.

So the way evolution works, the design we have works well enough that it doesn't cause problems. It might be the best possible design or it might not, all that mattered is that whenever it arose in evolutionary history it was either an advantage over what camebefore in terms of survival so propagated or it was not detrimental and paired with something else genetically that propagated.

We can't definitively answer your question but we can speculate on why it's a good idea to separate urine and faecal matter. Urine is a reasonable medium for growing bacteria. That wouldn't matter in the colon but would matter if bacteria from the colon could ascend into the kidneys and diarupt it's function. Valves could help or a bladder that drains into the colon, but complete separation may just be better.

It may also be that the acidic nature of urine would disrupt the helpful bacteria we rely on to colonise our guts to help digest foods.

Another possibility is the constant flow of urine would mean our faecal matter would never dry out. It'd be like having diarrhoea all the time and we'd need to poop constantly. The colon retrieves enough water - but not all water - that's why poop isn't hard as rock. If it was flooded with fluid it may not need to retrieve fluid.

The fluid might even be stuck in a cycle between the colon and the kidneys and make it harder for the body to keep homeostasis - as the kidneys excrete more fluid to try and regulate fluid volume the the colon could just resorb it. Basically the colon could end up working against the kidneys and cause even more work for thenl body. It may just be less efficient than discarding water as needed.

Drier faecal matter in the colon and a reservoir of fluid in the bladder does also give us freedom to release when it is safe to do so, which may protect us from predators (having to stop to poop even a few times a day is dangerous compared to only going when you know it's safe to as there are more opportunities to be attacked by a predator). It would also be very easy to track an animal that leaves a constant trail of poop and urine uncontrollably behind it.

All or none of these may be reasons why we have separate urinary and alimentary tracts; it's impossible to know and would always be speculation. But regardless these do seem like reasonable reasons why we may have separate tracts.

Not a biologist, just a normal dude. Isn't one function of urine to flush out toxins? With them staying in the intestines could it poison the human?

You're looking for a Cloaka

The other solution would be to evolve a bladder capable of reabsorbing water.

What if my mother had wheels?

The beargryllneys?