With Juxtapose, you can present images in “Before & After” image pairings. This is also a great way to compare differently processed versions of the same image, or for whatever other reason you like.
Edit an existing album template, or create a new one using the Theater Add-on. In the designer, under Presentation, set:
You will have several options you can configure, mostly self-explanatory.
With a template ready to use, go to Lightroom.
Under Publish Services, create a new album:
Drag images into the collection. The album MUST have an even number of images, i.e. images in pairs. Publish when ready.