The flipbook is responsive to the device layout. If the flipbook is showing up as a two-page spread on the desktop and as a single page on mobile (in portrait mode), then you can force it to be a two-page spread by adding the following code in the Resources section of the Export HTML5 with in5 dialog:
getOrientation = function(){ return 'landscape'; };
If you only want to force it to be a two-page spread on mobile, then you can add the following code:
var getOrientOrig = getOrientation;
getOrientation = function(){ return touchEnabled ? 'landscape' : getOrientOrig; };