Wiring the two sensor together may work to some extent, assuming that when the flow sensor switch closes it closes for a very short period of time. This is because during the time one switch is closed, it will mask out the other flow sensor. So if the closing time is very short, the total number of pulses you get is still roughly the sum of each sensor. But if the closing time is long, the total number of pulses will be considerably less than the sum. The best way to find out is to use a multimeter to check the conductivity of the flow sensor, and get a sense of how long the switch closes during each pulse.