I have 2 toilets which tie into a 3" stack. The first (main) is located approximately 3 ft from the stack and connected with a 3 inch line and sanitary tee. The second is located approximately 15 ft from the stack and connected with a 3 inch line and sanitary tee. The second ties into the stack approximately 6" on center below the main toilet. When the main toilet is flushed, the water is sucked out of the second toilet bowl. The vent stack is 3" out the roof and is NOT cloged. In the photos, the left side is the second toilet, the right is the main toilet. Up vents out the roof, down goes ino the basement floor and to the sewer. Thoughts on how to fix this?