Why did one toilet regularly clog, and now another one?
It's highly unlikely that the blockage transferred from one toilet to the other.

If your septic waste line is on a septic tank and leach field you may need to have it emptied.

If you are continually clearing blockage in your main septic line then you might want to consider having a plumbing service clear the main line. The reason I mention this is that it sounds like tree roots have infiltrated your main sewer line. To deal with these tree roots, specific tools are required. The problem could also be a restricted or broken line.

On the other hand, next time you are clearing the line (with a snake from the cleanout?), see what comes back attached to the spiraled head. If you are clearing it regularly, chances are you will see whatever it is attached to the snake.