Very interesting to hear of other's installations and experience.
Here is my system:
with typical engine in each stern. Came with 2 water heaters, each behind (slightly below) each engine.
Replaced both w heaters with single
one on one side, but with 2 seperate heat coils inside. Water hoses from opposite engine have to be routed maybe 30 inches above pressure cap of that engine.
When I installed this W H, I made sure to fill complete system of this opposite engine.Never had any problem over the last 4 years I guess. However, I recently read an article by a respected expert who said that this would court problems unless there is an expansion tank w/fill at highest point of system, including lines.
I looked into available expansion tanks
to do this, but finally decided to leave it as is. This thread convinces me that doing this is not too unusual or risky. I will need to make sure the system is kept full of coolant.