I think I can confidently say that between about 12am-5am, flow should be minimal, except the few services running sporadically like pool refilling and effluent systems...
Depends on your clientele - in some cases, at 12am, the party's just getting really started 
Now it makes it tricky...what sort of issues would arise attempting to use two interrupts concurrently with essentially the same program associated with it on a single Arduino? I'm guessing possibly a few lost pulses if the second interrupt occurs while the first is still processing?
This is why your interrupt routines should do as little as possible - in this case just increment a count. What frequency are the pulses from the water meter?