I just did a google search for twin heat exchanger marine water heater and got lots of results, including Defender Marine.
I took a quick look and I think your search returned models that have both electrical and a heat exchanger, not dual coils. Adding 2 coils would reduce the capacity of a heater or require a larger tank for the same capacity. Any tank with dual coils would need, 2 hydronic inlets, 2 engine heat inlets, cold water inlet, hot water outlet, pressure relief outlet, and may be a drain, a quick visual inspection of the tank would reveal if it had enough inlets and outlets.
It would be possible to add several three way valves to the plumbing and manually change them to heat from the hydronic heater or from the engine. That could be a PITA and prone to end user errors which could be damaging to the engine or hydronic system.