For InDesign CS6, you can include horizontal and vertical layouts in the same InDesign document. For InDesign CS5/CS5.5, horizontal and vertical layouts must be in separate InDesign documents. You cannot mix and match orientations within folio; the entire folio must be dual, portrait, or landscape.
A dual-orientation article has two layouts—vertical or horizontal. When the user rotates the mobile device, the second orientation is displayed. A single-orientation folio is locked into that orientation. For example, if you create articles with only vertical InDesign files, that orientation remains in place even if the user rotates the mobile device. However, if a folio is portrait-only, full-screen videos can still be played in landscape mode.