I have a house on a concrete slab with two toilets plumbed (I think) to the same pipe under the slab. When we bought the house one toilet always had a black stain on the bottom of it and if left unflushed it would develop a terrible smell like a public toilet. None of the local country plumbers could explain it so eventually I bought another bowl. It is slowly developing the same scum on the bottom, no matter how much we clean it and will smell bad if left unflushed for a day.
I am in a rural area and connected to an Earthsafe septic treatment plant so have been told I can’t use chemicals that are too harsh as they will kill all the good bugs in the primary tank.
My house is also located in a rural area having two toilets. The same problem occurred in my bathroom’s toilet. I consulted to a plumber. He suggested replacing toilet fixtures. On his proposal I replaced the old fixtures with the new one. Now the condition is much better and the stinking problem has also resolved.