A useful feature of the MFDs is you can overlay data onto most screens. You can have a virtual dashboard with all the engine info like speed, rpm, voltage, fuel burn, temp, etc displayed to look like analog gauges, but that takes up the screen. Split screens on anything below 10-12" size leaves both screens too small to be useful, hence the second screen networking capability. Two 7" displays usually cost less than a single 12" from the same product series...
But the data overlay option can help a lot. I have engine data showing full time on my GPS screen, so when I'm cruising I can see engine temp, voltage, water pressure, fuel economy and the depth under my keel at a glance. When I stop to fish, and switch to a sonar screen, this time the overlay data is water temp, battery voltage and speed over ground. I find most of the time I don't need to use split screens.
I got along OK for 5 years with a single HDS 7, just bought a second hand HDS 10 to help out the old eyes. Might move the 7 aft and network them to share transducer.