Do you see the water go into the bowl …. because to be honest it sounds like the water is being siphoned back into the water main. The reason being, i cannot fathom how flushing one wc will cause a second ones cistern to lose a small amount of water. Of course it could just be a coincidence and the second cistern is running ever so slow into the bowl.
Lift the lid on the second cistern and see if the top inch or so is still above the water level. If not that could be the problem. The second cistern’s inlet valve is leaking slowly, and has filled above the inlet valve. when the first cistern is flushed the pressure in the mains drops and sucks water back into the mains from the second cistern. Then as the pressure in the main increases it refills the second cistern.
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or
will be taken. And yes, I am a licensed plumber with my own business in Brisbane Australia