The drain from the toilet to the main drain is plugged and the branch to the shower is not. If the main floor toilet flushes, it is between the two toilets and not between the house and the sewer.
partially clogged drain or vent, improperly installed vent or drain
Grey is usually dirty water but not toilet water (shower, sink drain) Black (or Brown) is sewage from the toilet. If you have a motorhome there will be two sewage tanks... "grey" and "black"
yes, by eating it
Hydraulic shock from quick closing valves and can cause piping to rupture Hydraulic shock from quick closing valves and can cause piping to rupture
Yes, as long as you seal the drain of the toilet you are removing.
The drain is plugged somewhat between the upstairs and downstairs. Need to snake the drains.
yes they are
If your plumbing system is properly installed there should not be any problem. If it isn't installed properly, you would already have water from sinks and bathtubs backing up out of the downstairs toilet.
The sewage pump out is installed downstream of a bathroom group from the upstairs plumbing Consult a plumber and/or the plumbing code. If you do this wrong you WILL regret it later. Keep in mind you are dealing with raw human sewage. Do it right the first time, for your own sake.
partially clogged drain or vent, improperly installed vent or drain
Answer The waste line from the downstairs toilet is not vented (or not vented properly, or the vent is plugged up). Flushing the upstairs toilet creates a vacuum in the main line which sucks the water out of the one downstairs. Answer your waste lines for both toilets may be on the same branch off the sewer main. Either you have a stoppage in the branch line or you have a stoppage in the main line. (If it's the main line your downstairs toilet would have to be the lowest fixture, ie If you have a floor drain downstairs and it's a stoppage in your main line it would backup through the floor drain. If there is no floor drain the toilet downstairs is probably your lowest fixture.) Your best bet is to snake out the system through the toilet downstairs.
Grey is usually dirty water but not toilet water (shower, sink drain) Black (or Brown) is sewage from the toilet. If you have a motorhome there will be two sewage tanks... "grey" and "black"
There is a main drain that runs from the basement through the roof where it is the vent for the system. If you look at the layout of the house, the upstairs bath is most likely over the downstairs one or the kitchen. If you look on the roof, you will see a 4 inch pipe coming through the roof that should be over the upstairs bath. The sink and shower drain into that and then to the basement and out of the house. Most likely the main stack is in the wall behind the toilet.
Unless it is a very strange arrangement, turning off the toilet on the first floor should only shut off the water to that toilet. It should have no effect on the upstairs shower.
There is a blockage in the waste line. Have you flushed the basement toilet to see what happens? Try running the upstairs shower and see what happens. (use two persons , one up stairs, one downstairs so you can control the amount of water that rises in the toilet.) There should be a "clean out" cap somewhere downstairs near the point where the waste line exits the house. It is here that one would "snake" the line. The other possibility is the water level in the downstairs toilet is lower than the level of the waste line exit. This means that the point of exit of the line is physically higher than the toilet. The only remedy for this is to raise the toilet to above the height or replumb the toilet into a waste pump that evacuates upwards to a point slightly higher than the waste line. This is the most common set-up in a "basement" toilet. The toilet evacuates into a point lower than the floor. The waste pump or grinder pump then pumps the waste upwards than gravity allows it to evacuate normally. The benefit is that the waste pump creates a separate system for the basement toilet. Y-THINK-Y
A portable commode is often used when a patient is sleeping downstairs, and can not get to the upstairs toilet (which may be the only toilet in the home). Usually, the commode is designed to look like a piece of furniture. If the patient recovers and can use the upstairs toilet once more, the commode can be put away. Being portable, the commode's bowl can be carried upstairs and the contents disposed of down the toilet and into the sewerage system. Portable commodes, chemical toilets, and such like, are often used when camping, or caravanning, or if the house is not connected to the sewerage system.
A down line clog would cause this. The shower drain is lower than any other drain in the bathroom so it is the first to tell you of a clog in the line. Time to Roto Rooter! Y-THINK-Y The toilet and shower must have the same drain. There is a blockage somewhere after the toilet and shower drains meet. The backup takes the easiest route back which must be the shower drain. Try snaking out both drains. Start with the toilet. To do the best job, remove the toilet and start from floor level.