The game simply uses what it is provided. In order to use more than one monitor, you must make the monitors appear as one monitor to the game. This is usually accomplished in the video drivers configuration.
Once you have the monitors appearing as one monitor to the game, it is a simple matter of changing the game resolution to match the accumulative resolution of the monitors.
For example: If the two monitors are natively 1920x1080, then the game resolution, for two monitors, would be 3840x1080, if the monitors are configured side-by-side.
If they are stacked, then 1920x2160 would be the resolution.
If they are side-by-side, and rotated, then the resolution would be 2160x1920.
If they are side-by-side, rotated, and stacked then the resolution would be 1080x3840.
Hope that helps.