Re: one or two batteries..???
Do you NEED two or do you WANT two?
Spending long hours on the water with accessories burning up voltage such as lights, radios, amplifiers, fishfinders, GPS/chartplotters, depth finders, etc? Then having two is a necessity. Don't have any of that stuff and engine starts just fine after 10 hours on the water? Then two would be a benefit, not a necessity. You didn't indicate whether they were both cranking, deep cycle, or one of each.
If you already have the batteries, I would install a 4 position battery switch (1,2,both,off) and invest a little more in a battery tender. It's a great piece of mind to have the back up starting battery and you already have the most expensive part on hand.
