Downstairs? First floor with a basement or in the basement? If first floor with access in the basement or crawlspace, no real problem. PVC, just cut the pipe and install a T. Cast iron, you can use a Saddle Clamp made for this. Looks like a PVC T split length wise and held to the cast iron with two U bolts.
If adding a bath in the basement, you will have to bust out the floor, add the drain and replace the cement.
Unclog drain that both sinks drain into.
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.
What makes a bathroom drain smell, soap makes a bathroom drain smell
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.
Because they share a common drain line which is clogged. Gravity and a common drain pipe means the back-up would appear in the downstairs drain, even if it appeared to be cleared in the upstairs drain. Be glad you are not just slightly uphill from the clog. The sink line line needs to be roto rootered from upstairs, through downstairs, through to the common basement drain to clear out the clog.
Pee in the drain or sink! n00b
This could be caused by your exhaust fan not properly removing moisture from your bathroom. It is always a good idea to use some X-14 (a mildew remover) every coupke of weeks when you start to see mildew. It is cause by lack of air flow in your bathroom, or a leak from upstairs. Is there is an upstairs bathroom? If so you might have a small leak somewhere in the supply or drain lines in the upstairs bathroom. If not a leak then possibly condensation is forming on a shower or tub above this bathroom.
It should be on the lowest run of piping on one of the downstairs radiators. Or you can just drain the system from one of the downstairs valves (carefully). Or it might be behind the fridge, that's where I found my tape measure.
A vent for the drain system may be blocked. A home or building plumbing system must be "free flowing" with a vent to the outside atmosphere - usually through the roof. When water is forced down the drain and the vent for that drain is blocked, air will be pulled from inside the building through another plumbing fixture causing the gurgling sound. The main drain line may also be partially blocked whereas when water is forced down the drain the air in the drain line may be forced back into the building through another plumbing fixture causing the gurgling sound.
Bathroom or kitchen should be square and level. The plumbing drain lines need slope to drain properly.
If this is a new occurrence (it did not happen in the past), the most likely cause is a dry drain trap in the downstairs bathroom. A drain trap is a double U shaped arrangement for the drain that allows water to be trapped in the U-shaped connection. The purpose is to form a water plug that prevents sewer gasses from escaping back through the drain hole. When this water evaporates, the gases can get through. You can see this arrangement if you can view the drain pipe under a sink. It is visible under most bathroom and kitchen sinks. To solution to the problem is to dump water down the drain to fill the trap. This occurs most often in unused bathtub drains and floor drains. You may need to look under a floor rug to see if you have a floor drain. If you have a floor drain, just pour about a quart of water down the drain to fill the trap. If this does not solve your problem, you may need to call a plumber to check out what is called the vent stack to see if that, or some other issue, is causing the problem.
The main drain is plugged. Not completely, but enough that it is easier for the upstairs water to come out in the shower and floor drain instead of going out the drain.