I don't expect to see anyone selling monitors capable of both FreeSync and G-Sync.
Technologically capable, yes, through (warranty-voiding?) aftermarket firmware hacks and mods.
Legally capable, no, not with NVidia's heavy investment into (and restrictive licensing/control of) G-Sync.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
[/Korth]