With OS X 10.11 El Capitan, it's easier to view two windows side by side with Split View. 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.
In Photos, click on a thumbnail to expand a photo from your library. The Split View button is located in the upper-left corner next to the back button. It opens a narrow panel along the left that shows thumbnails of the other photos in the collection you are viewing. By default, the left panel has two columns of thumbnails, but you can drag to resize it.
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. You can also use the up and down arrow keys to move through the grid of thumbnails. When you reach the last photo in the group, the right arrow will take you to the next collection of photos in your library.
(Via OS X Daily)