Depends on what youre trying to see and how much information you want. As long as youre looking at full screen, a single unit is great — but with the “want” to see more, Ive seen guys split the screen, either into 1/2 and two 1/4s, sometimes two 1/2s and sometimes 4 quarters. Each time you split, then you reduce the “seeable” side of the unit. With two units, you can split one to show sonar (only need a screen about 2″ wide as everything you see except what is coming off the right side is history) and the rest dedicated to Mapping. Then use the other to see either SI or SI/DI. Give a larger viewing area. This is the same reasoning that so many office workers have gone to dual or triple monitors instead of one large monitor. Youre the only one that can decide how much information youre wanting to see.John