You can solve this with several methods:

1) Multiple engine instances, each with a different fullscreen monitor (Pro only)
2) Desktop spread over all monitors, multiple engine instances, each with a different borderless window of the size and position of one of the monitors (Commercial and above)
3) Desktop spread over all monitors, single engine instance with one window and multiple views of the sizes and positions of the monitors (all editions)

1) and 2) support different resolutions. 3) only works on a graphics hardware that supports rendering a single DirectX window to several monitors.