Before we purchased our Navigator, I nearly opted out of the Rear Entertainment system because of all the negative reviews. I'm so glad we didn't....Once I sat down and really dug into the system, I have grown to love it. it's FANTASTIC and here's why... First, in response to only one monitor having the SD slot, that's all you need. The 2nd monitor can access the SD card as well AND play a completely different movie. Screen A and Screen B are interfaced together so that Screen B can access everything on Screen A simultaneously or independently. I have about 30 movies loaded onto our SD card (loaded in Screen A) and sometimes the kids watch the same movies and sometimes they watch different movies at the same time. If the other child decides to join the movie being watched on screen A, he can easily switch and now both screens are playing the same movie at the same time in unison. He can even watch the same movie but start it from the beginning so same movies on both screens but not playing in sync. It really is that intuitive.
In addition to all those movies on the SD card, I purchased a Roku stick and that is inserted into the HDMI port of Screen A. It is powered by the USB cord, which also is plugged in to Screen A. And FYI, the USB power for the Roku stick from Screen A is more than enough. The kids have used it on a 7+ hour trip before. Screen B can also access Screen A's HDMI port simultaneously or independently. So if Screen A is displaying a SD card movie, Screen B can access the Roku stick. And with the Roku stick, there's Netflix, Google Play movies, Vudu, Amazon Prime videos and much, much more. You of course need subscriptions to Netflix and Amazon prime and some others but I would imagine most have both or at least one. You don't have to pay additional to access these if you already have a subscription. Just simply sign on just as if it was another TV in the house. That's an additional thousand or more titles to select from. I set the kid restrictions on all of them so they only have access to kid's content.
The best thing, it's a clean interface. We don't have tons of DVD movies cluttering up the vehicle anymore and on those nice big screens, the backseat passengers love it. So with the SD card and a Roku stick alone, you literally have thousands of shows/movies to select from. And if you have a Slingbox at home, there's a built in Slingbox app integrated into the system, so now add in all your live TV and DVR access on top of the other thousand selection of titles!
FYI, for the Roku stick, a Mobile hotspot or the vehicle hot spot is required to access content on the Roku stick apps. We are fortunate to have unlimited data hotspots through work, so this is what is used to provide WiFi to the Roku stick.
Now the only hurdle we ran into was our older 3rd child who sits in the 3rd row. Since there is not a screen for him, he uses his tablet which has all the same Roku apps. So he can access Netflix, Amazon prime, Vudu etc on his tablet. There was another post where someone asked which tablet mount was good for the 2018 and up Navigator. I made a reply post with some info on what I used. You should note that the distance between the the two post in the headrest, is not that wide. You'll be hard pressed to find a tablet holder that can fit in that space. I had to find a holder that latched on to one post. And I searched relentlessly looking for the one that spring loaded into the two post. If anyone that found one that does fit the newest Navigator model, please post here.
I know this has been a long post but I'm hoping this helps some folks that are on the fence about the Lincoln OEM rear entertainment system. I think most of the negative reviews was mostly due to the fact that so many people couldn't figure out how to use the system. Even the Lincoln sales rep don't have a full understanding. But once you do figure it out, the options of all the content that's available, is incredible. Every in vehicle entertainment system I've had before, was always limited to the number of DVD we had in the car. Now, our choices are endless!
Rich
I am also thinking a Nintendo Switch can be attached the same way, interchangeably. I am a little worried about sticking something with 3M tape to the back of the seat, but hoping it would just blend in, especially with the black trim ring it has as an option. As an added benefit, I could not keep the tablet in the car most of the times, therefore not attracting thieves. I wonder if the tablet would fit in the seat pocket, for a certain broken screen experience at some point 
