I've obviously got a much smaller boat than what you're looking at, but imo I think a lot of people go with two smaller screens for a few reasons, of which the first would be the layout. On my boat, it has the little electronics bin behind the steering wheel with the flip up clear tinted visor attached to it. With that on there, it doesn't give you much room to put anything bigger than maybe a 9-10" screen inside the compartment? I don't personally know, but I bought a Garmin 943xsv to put in there and while I think a 10" screen could probably fit in there, I'm not sure if a 12" could, although I could obviously be mistaken. I went with the Garmin 943xsv for several reasons, the price, the ability to add radar later on if I wanted to, and it has all three sonars available to use on it without needing a separate black box. I know guys with my boat will cover up the compartment with starboard or something similar and flush mount larger electronics but I didn't have the time to do that as my old Garmin that I bought the boat with crapped out on me a week before the opener for the fishing season. I think you also get much more "bang for your buck" with having two MFD's in that a new Garmin 943xsv is $1,241 on BOE Marine's website yet a 1243xsv is $2,370, or almost double the price. So, if you like the idea and look of two screens then you get two 9" displays for a total of 18" as opposed to a single 12" display. The prices only go up from there with regards to the higher end Garmin 12" or even 16" displays, their 8618xsv is $4,800. So you could buy two 9" screens for half that price or two 12" screens for almost exactly the same price and get more "real estate" so to speak.
I personally don't really have a preference, I've been on boats where I used a single 12" or 16" screen and I've been on boats where I've used two smaller 9" or 10" screens. If I had to choose one then I'd probably go with two 9" screens because I'm more of a frugal fisherman/boater and don't have all of the fancy things like radar as I don't know if I'd ever use it. Maybe one day I'll buy it for my boat, but I keep going back and forth on the matter as I honestly think there have only been a handful of times or less in my almost 40 years of being alive where I was out on the water and wished that I had it. I do think that two screens is probably more of an "old school" type of thing where you had dedicated sonar or radar units and had to have 2-3 different displays or more just to get what you wanted. I also think a lot of those people probably got used to the idea of multiple screens so they stuck with that look or approach when they eventually upgraded their electronics. Or, they had the holes already cut out in their dash or holes drilled for the mounts and wanted to go with the least intrusive route as possible, so they bought two new units. I will say, that If I had radar and/or did a good deal of reef fishing or looking for structures then I'd probably want 2 displays. That way, I could run one on one screen and then the second on the other with it zoomed in as much as possible. That way I wouldn't have one screen divided up into 3 sections with trying to be zoomed into the max on say the SideVu sonar looking for structure and then have my chart plotter or radar zoomed way out. Just my opinions, of course.
