You can span the display using Nvidia's Surround, which is part of the Nvidia control panel.
There is something similar to this if you are using an AMD video card.
At present, FS2020 DOES NOT allow you to open new additional windows for separate instruments, views, etc., like it did in FSX.
Hopefully Microsoft will address this in the future, and make this benefit available, ... although I doubt it, because the software is designed to mainly run under an XBOX single window environment (using either a PC as in XBOX Beta for PC or on an XBOX console.)
I'm currently running FS2020 using three screens with a resolution of 5760 x 1200, and FS2020 runs fine across these three monitors which are in Nvidia's Surround (so it looks like one big, very wide screen monitor.) And this works great, EXCEPT it would be nice to be able to open up another window for just the instruments, like I could do under FSX.
One of the trade-offs, of going with the much, much, much more advanced FS2020 software.