Hi

Okay, here goes...
Septic tank pumped about 10 weeks ago.

Back to back bathrooms.

In bathroom 1, after bathing and when tub is draining, water has just recently started not draining out of the house to the septic tank, rather it backs up somewhere and ends up coming out between the bathroom1 toilet base and floor and also then seeps through the shared wall and onto the floor of bathroom 2.
This does not occur every time the tub is drained, more like 60 or 70% of the time.
Bath 2 tub has no draining problems, but if the toilet in bath 2 is flushed while tub in bath 1 is properly draining, flush will instantly cause the draining bathroom1 tub to leak in the manner described above.
Just yesterday, flushing the toilet in bath1 caused leaking and some mighty gurgling from drains of tubs in bath1 and bath 2 along with some "stuff" (not poop or anything, but looks like mold or mildew or something) coming up thriough the drains of both tubs.
These are the only two bathrooms in the house, and no other drains have any problems.

This is a site built single family home on a monolithic slab on grade foundation. Home was built in 1964 and purchased by us in 1998 with no previous plumbing issues prior to this mess.

Any advice or cures would be much appreciated.

Thanks
Paul
High Springs, Florida

Surprise!!   :o  I'm lead plumber on three Q& A sites.   :D  so I guess the answer would be, Hi Paul, Greetings from Sarasota. If the tank's OK then you have a partial blockage in your drainage lines. With a partial blockage the water would still drain but when you increased the volume of flow,(like flushing the john) it would back up. The next step would be to find out where. Locate the sewer cleanout. It should be about 18" off the foundation and brought to grade or just under the dirt. Open it up and flush the toilet in bath #2 while draining a tub full of water from bath #1. If the water backs up out of the cleanout then the problem is in the line between the clean out and the septic tank or the septic system itself and you will need a septic tank company. If it starts to back up in the house, that means the blockage is under the slab in the drainage lines. You now have two options. If you call in a plumber you have saved the labor expense of him having to locate the clog. Or you can rent a sewer machine and go up on the roof go after it yourself. Run the snake down the vent from the roof vent of the furthest vent. Run it past the cleanout that you opened up. That should clear out the bathroom branch and the house main.  Good luck Paul.  8)   Tom