Thank you all for your input.
Rick - Great picture - It's like being there (if only ...)
As mentioned earlier, I have started a further thread, "one lump or two?"
on the basis that the justification for either course of action was unclear.
Having just looked at the Yamaha site, which includes "Performance Bulletins", I was amazed to see the relative fuel consumption of two engines, as opposed to one, which puts the "economy" of 2 x "conventional" four stroke 60's not much different to the 5.7 litre V8 fuel-guzzling monster, which lurks in my present boat.
Please have a look at my other thread, for some eye-opening statistics!