I'm using two flow meters in a radiant heat system. I measure them on adjacent pins D7 and D8. If I have an input on one and the other is floating, they both count. Surprisingly, the floating pin counts very close to the one with the input. A pull-up resistor of about 7K on each of them solved the problem.
Hardware is a Pro Mini on a breadboard. The flow meter has a clean Hall Effect output.
Just for info.