Don't flush the toilet while someone is taking a shower. your septic tank may need pumping off if you have one..if you have city sewage sounds like the drainage line is stopped up so the water comes in the tub before it can exit the clogged drain..hope this helps..
The drains for the shower, toilet and sink all connect to a common line. Assuming the toilet is not backing up, the problem indicates that there may be a partial blockage, enough so that some of the toilet flow is backing up through the shower drain line. You need plumbing repair. The shower leak you mention should only be in the shower, or else you have a broken drain line as well.
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.
I can be but the toilet needs to have 24" in front of it to be legal. Plus you cannot tie your shower drain in the the horizontal branch of the toilet.
This can be a complicated job as the toilet drain ranges from 3" -4" and a variety of materials from Plastic to cast iron to galvanized. Then there is the venting to consider and NO a shower drain CANNOT be used for a toilet discharge as the shower drain is normally 2" diameter
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.
Because the drain is plugged downstream
A shower drain is 1 1/2 or 2 inch and a toilet needs to be either 3 or 4 inch.
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.
You need at least a 3 in. pipe -- shower is 2 in. pipe
Your drain line is plugged somewhere after your shower and toilet line are connected. The reason for the gurgle in your toilet is because air is trapped in the drain and your toilet is the closest place for air to flow to. If you had a vent on your toilet trap arm then it would gurgle in the vent rather in your toilet but your drain is still plugged and needs to be snaked out and maybe your drain line needs to be examined for right grade or broken.
If the stool is the lowest point of plumbing in the house it could be several different reasons. If stool is not lowest and the lowest drain point is not backing up, then you have a plugged toilet.
If you reduce the soil connection to 2" and install a trap