domestic plumbing

[mtw]

Ever come across this problem?
If a wc is flushed, the water level in the adjacent wc bowl rises slightly - sometimes bringing with it foul matter?

 

[PhilZ]

The closet that backs up is lower than the fist, and waste is relieved at second fixture. Check for blockage and pitch after lower fixture.

 

[KSherlock]

I would guess the venting system is either too small or blocked. Air pressure in the system is increased by simple displacement thereby moving the liquid in the adjacent bowl.