Why 2 thermostats in Yanmar 3QM30H?

[Brazil]

I have a 1980 Hunter 37 cutter, with the Yanmar 3QM30H, designed for seawater cooling. The manual shows only one thermostat, but two openings. When I opened mine to check, there are thermostats installed, one each fore and aft port. The manual, however, shows a thermostat only in the forward port, and the Yanmar parts list shows only 1 required. Do you know anything about the need or utility of two separate thermostats in this application?

Also, my boat has an aftermarket conversion to fresh water cooling, with a Jabsco pump bolted to the lower, port side of the engine, what appears to be a Sen-Dure heat exchanger, and a Raritan hot water tank. No thermostat or zincs in this circuit, only in the engine's fresh water circuit. Do you think this is original from Hunter?

 

[P100D]

Diesel truck mechanic here. This is common in diesel, 2 thermostats. Most are even at slightly different temps. 1 opens and coolant flows if heat exchange, radiator or exchanger for boat, is sufficient to not increase temp then sufficient flow is reached. keeping the engine at optimal temp. if not then second thermostat opens creating more flow and more heat exchange.