If both monitors are 1920x1080, you have two options:
- 3840x1080 (side by side) or
- 1920x2160 (one on top of the other)
To do 3840x2160, you would need four displays.
Eyefinity is for managing display layouts and will work with any number of displays from 1 to however many you can plug in and use at the same time.