Many flats have the pipes for upper flats passing through the lower ones.
I found a flat six years after conversion where two flats shared two meters!
So the only way to turn off gas was at both meters and thus turning both flats off.
Just shows no one had ever done a tightness test even when they were being converted or any time until I got there!
Tony
Do you mean that somewhere the pipes in both flats were connected, or both flats had appliances running off each meter?
Did you rectify?