I noticed this the other day on my TV, but failed to realize that it is a new feature!
Chromecast and Google’s smart displays will now show portrait photos side-by-side. Earlier, they were displayed individually, with the same photo as a blurred background to fill the screen. That was kind of neat too!
Yesterday I noticed two portrait photos of my daughter side by side on the TV. However, I thought I might have created a collage using Google Photos with those two photos. Today I noticed this story on Android Police and quickly remembered these two photos and checked my album. They are individual photos.
Google has since updated their help article and made this official:
Portrait photos are now shown in a side-by-side format, with two related photos. Google Photos uses a number of signals to relate photos, including if they have the same person in them (in territories where face grouping is enabled), were taken close together in time, or were taken close together in location.
Just to make it clear, this is for photos that are taken holding your phone vertically, and not those “Portrait Mode” photos with the bokeh effect and all that fancy stuff.