You can still use the left and right arrow keys in Split View to navigate from photo to photo, but the feature lets you click on a thumbnail to jump to any photo in the group. By default, the left panel has two columns of thumbnails, but you can drag to resize it. It opens a narrow panel along the left that shows thumbnails of the other photos in the collection you are viewing. The Split View button is located in the upper-left corner next to the back button. In Photos, click on a thumbnail to expand a photo from your library. The new Photos app also has a Split View button, though it operates a bit differently than El Capitan's system-wide Split View feature.
With OS X 10.11 El Capitan, it's easier to view two windows side by side with Split View.
