Dan , do a search on single vs twin set ups ! I suspect that the short answer to your question is that there are more cons than pros in a twin engine set up ( weight , cost , maintenance etc ) . Where do you draw the line in " back-up " . Twin engines , twin batteries , twin fuel supplies ? I have seen such set ups on Ribcraft 585s , but they have always been safety boats or some such like . In my opinion , a well maintained 4 stroke , perhaps with an auxillary is a better way to go !
You mention serious cruising . How well prepared are you and crew ? Never mind the boat . Have you considered cruising in company as a seriously safer way to take on such cruises ?
At least your off to a good start with boat choice , but that debate is just going to kick off again........( possible change in build method since they landed big Olympic contract !) Good Luck Bern