The page spread was introduced to enable you to layout images and other objects outside the viewport, and eventually bring them in via the actions (with an animation or a move, for instance). You can set this both at a project level: under General Settings the Page Spread property is set by default to auto - you will have automatically a Page Spread double the size of the viewport. Otherwise you can customize the Page Spread value to your needs; you can also set this value at a page level: go to the Page Inspector, under the Workspace tab you will find the Page Spread property.
In order to have two spreads in a unique view, you need to set the Single Page option to false, you will find this under General Settings -> Appearance and Behavior. In this way you will have your output (on iBooks, for example) with your even pages on the left and odd pages on the left - for example page 2 - 3 will stand one next to the other.
