I don't have an official answer to your question, but I have experienced some pain with this topic!
I ran a G-Sync monitor and a Freesync monitor, with Freesync disabled. You would think it would work, right? It didn't. I can not be specific on what failed but I instantly noticed that the frame rate on the G-Sync was stuck at 60Hz despite the fact that neither monitor was a 60Hz monitor. It may be that having two identical monitors will be fine but I don't know, I would just recommend extreme caution.There seems to be no information out there about two adaptive syncs at the same time. Make sure you buy from a company that will accept no question returns.
I should add that I have non-adaptive sync monitors and they work fine with a primary G-Sync. And also I have heard some people say that the NVIDIA app can get confused if you set G-Sync to Window mode. This is because it only has global G-Sync settings which obviously doesn't make much sense if you have two different monitors. But to me Window mode is important because I want to be able to swap easily between the two monitors, so Fullscreen mode is not something I would want.
Anyway, in the end I used a non-adaptive sync monitor as my second monitor. You will need to experiment with yours to find out if it can do the specific task you want it to do.
Let me try to explain that better. If you have two g-sync monitors, and you are running two apps, one on each monitor, the nvidia app only has one global setting for g-sync, so which app does it apply the settings to? if you have g-sync set to "fullscreen" mode it applies the settings to the app that's running fullscreen. If you have g-sync set to "fullscreen and window" then it applies it to all apps. You see you have a number of problems in that. Of the various combinations of fullscreen and widescreen for the apps you are running you can still hit problems no matter what the settings are. When you have say a fullscreen app running on monitor 1 and a fullscreen app running on monitor 2 the g-sync can get confused and when G-Sync gets confused it seems to limit refresh rates to 60Hz. The g-sync configuration really is a bit naff.