Ulysses is fully compatible with iPhone Duo. You can use all of Ulysses' features on either screen, including writing in the editor and exporting your documents. The app adjusts automatically as you open, close, or rotate the device.

When the iPhone Duo is closed, you can use the outer display. 

When it is open, Ulysses adapts to the larger inner display and supports the device’s different orientations.

Ulysses follows the system conventions of iPhone Duo. The navigation bar, which is usually at the top, and the toolbar, which is usually at the bottom, move to the vertical bar on the side when the device is closed or opened in landscape.