It IS possible IF your application is fullscreen over the entire X screen, a single X screen; but among other things, that requires all monitors configured to that X screen to be G-Sync capable. Identical monitors seems like a very good idea here.

So you shelled out for two G-Sync monitors, say dual 24" 16:9 1080p 144Hz. Side by side perhaps? What sort of games will you effectively play on that, with a border between them (regardless of "thin" bezels) and a super wide 32:9 aspect ratio? If you thought getting ultra wide 21:9 working on a single monitor was bothersome, you're in for some tough lessons from reality. Maybe you can live with the border in the middle of your games.
Or get three monitors... and now your setup is simply stupid, expensive and impractical for the vast majority of games in existence. Sure cool to run F1 racing games on that 5760x1080 setup! Let's hope your PC can handle it, and your eyes somehow evolve super wide peripheral vision!

Maybe you can flip all three on their side and have a 3240*1920 setup. Huh. Why don't you just buy a 4K monitor instead.

So perhaps the better choice is to go with a bigger monitor, maybe ultra wide 21:9 34" 3440x1440, but then you run into the problems of them being expensive as a mofo. Plus, that's not multi-monitor is it?

So then you think, hang on, did he say single X screen? Yes, thanks X! You ancient #%&§#¤ piece of computer history ¤%&#& omfg why are we still using it argghhh
Just use dual X screens! Yes, perfect! Piss right off with that happy hippie crap. Getting dual X screens working correctly is easier said then done. You're welcome to try.

So what we are left with are the last resort options.
1. Disable other monitors to get G-Sync working.
2. Have no syncing at all and get tearing/stutter/flickering etc, flip a coin.
3. Mess around with forced composition pipelines and V-Sync and just hope it's good enough.

In my personal opinion those choices range from unacceptable to barely good enough while still performing well enough.
Thanks Nvidia.