I have two water heaters - how can I tell if they are set in series, parallel, or are isolated?
If either heater has a shutoff valve in the incoming cold water pipe, turn it off and see which hot water taps/washer/dishwasher work and which ones don't. If they all continue to work when one heater is off then the heaters are essentially parallel. But since they are in different areas I suspect you'll find a split - possibly one for all bathrooms near the bedrooms (eg to always have hot showers) and the other for kitchen and laundry.