Because of the aspect ratio of most displays, when creating a slideshow of my photography, I typically show a single image when it’s orientation is landscape, and two images when the orientation is vertical. Although it may be more work to find images that work well together, displaying two vertical images better fills the space, provides the opportunity to show more images, and helps change the rhythm of the images in the presentation.
To create the diptychs, sort the images into pairs (I prefer using collections to do this). Then, in the Print module, create a template at the correct size using the Print Job panel (19.20 x 10.80 at 100 ppi for HD videos for example).
Select the images, and choose Print To File. Import the new diptychs into Lightroom, reorder as needed, move to the Slideshow module, and you’re on your way to a more interesting presentation of your work!