The water lines to and from each of the two water heaters originally on our cat were connected together (hot to hot and cold to cold only, of course) at one central point . So even if only one engine
had been used, hot water was available throughout the boat
. It would, of course take longer for hot water to flow to the opposite side.
Water temperature fluctuated if only one engine
had operated, so you might want to have a valve at each water heater
to isolate it when desired.
If you will allow me to ramble... I later replaced the two water heaters with one larger one that had two heat exchangers, and plumbed coolant
from each engine to that one WH. Temperature still fluctuates, but not so much.