Each time someone showers in the master bathroom, the water comes up in the second bathroom tub. And, it goes the other way around too. With the water, comes dirt and leaves, which appear to be coming from outside. I have used a plunger, but the water remains in the tub. Now, both tubs are filled with water, dirt, and leaves. What is the problem?

Plugged plumbing, I would Take a 5 Gallon bucket of water, And pour down Vent in roof
Quickly, to dislodge obstruction. May Need to use snake. I would start from roof.

If the blockage's downstream from the two bathrooms then pouring water down the vent will just let it spill out of the nearest fixture with a drain close to the floor. You need to have the lines snaked out. Sounds like the mains on bath #1 and 2 need snaking. Yo may go down the lavatory roof vent or pull the toilets and go down the open closet bend. Good luck, Tom

Am not a plumbing expert, it was what I would do. Has worked before.
I had a stoppage of my sink drains, I took a Shop Vac with several attachments, about 20',
Duct taped them together, put down vent and turned on, looked in Vac and found pile of Rust scales, didn't find any Diamond Rings.
Has worked great ever since.