Ive always had dual battery and charger set-ups but recently went over to a single battery and just carry a spare jump starter instead Batteries Direct
I was recommended to go this way from an outboard mechanic who makes nothing from the jumper system and heaps on setting up dual systems for those who insist on them.
This has proven to be extremely helpful as Ive already used it at sea to jump start another boat and twice at the boat ramp on other peoples 4x4's who have left their lights on all day flattening their battery.
Have a look the NoCo Genius Lithium Battery Jump Starter Pack as this type is carried by many of the marine rescue groups over here, some can jump large trucks and tractors.
I f you want to go with the expense, maintenance and weight of dual systems then go ahead. Ive also pulled the very expensive dual C-tec charging system and large extra battery from my 4x4.
By the way when I fish right through the night I tend to use rechargeable led lights and have most of my boat electronics turned off. In 40 years of owning boats Ive yet to have a flat battery or break down, not bad considering Im spending 250 engine hours per year on my own boat and around the same on other boats both recreational and commercial.
Ive also been a volunteer member of the marine rescue and found flat batteries are not that common here, maybe the cold your side of the pond might make some difference. Keeping a battery fully charged and well maintained is also much easier on my boat kept at home and though I do own a 80 watt solar system its never been used since buying it 5 years ago.