Normally, the flipbook exported from InDesign with in5, is responsive to the device layout.

It will show a two-page spread in landscape, and a single-page in portrait orientation.

Because in5 is extendable, you can override this behavior.

If you attach the following code as a .js file in the Resources section of the in5 dialog (how to attach resources):

getOrientation = function(){ return 'portrait'; }; 

That will force the Flipbook to stay in single page mode.

To do so, instead, only on mobile, you can attach the following code:

var getOrientOrig = getOrientation;
getOrientation = function(){ return touchEnabled ? 'portrait' : getOrientOrig; };