I have a 2 storey layout house. The guest restroom on the ground floor is exactly below the Jack and Jill Washroom of the top floor. It looks in both the restrooms the toilets share the pipe line

The problem is when someone goes to restroom on the ground floor , the top floor toilet gets filled up with **** and looks like the water when flushed downstairs sometime rushes up.

What could be the problem.

Is there a simple way to find the actual source of the problem.

Any estimate on what would be the cost of fixing this problem.

This is also true when someone uses the toilet upstairs, the water even after flushing when its clean after sometime gets filled with the stuff and it looks like the person didn't flush the toilet after use.
Any kind of inputs appreciated

One of the first thngs ab new plumber is taught is that liquid will not run uphill unless it has pressure assisting it. This is a universal law of physics.(Water seeks its own level.)
You would seem to have found a way to get around that law.
With a blockage in the main the lower toilet would backup when the upstairs toilet was flushed, It's not supposed to work the other way.
I've taken this long to say. " In over 50 years out in the field I've never run into this problem. I haven't a clue."
Since plumbing drainage works on gravity your toilets have apparently repealed the law of gravity.
It's going to take someone on site to figure this one out and I sure wish it was me. If you live anywhere around Sarasota Florida I'll be more then happy to make a no charge service call. If not please keep me in the loop and let me know what you find out. Regards, Tom