By default, the flipbook exported from InDesign with in5, is responsive to the device layout in which it shows a two-page spread in landscape and a single-page in portrait orientation.
You may have seen the code used to modify the behavior to force the flipbook to single page mode in the following article:
Similarly, the code from that article can be modified to force the flipbook to only display the document as two-page spreads by attaching the following code as a .js file in the Resources section of the in5 dialog (how to attach resources):
To force the document to display the two-page spreads only on mobile, you can attach the following code: