To review, rate, and crop photos side-by-side (2-Up) use the preview window. 

From a contact sheet, select a slide. Use either the magnifying glass icon or command-R (macOS) or control-R (Windows.) (R for "review") 

From the preview window, select either horizontal or vertical orientation (keyboard shortcuts and V.)  Select images to compare by selecting them in the filmstrip preview or by using the arrow keys. Return to 1-Up preview either by selecting the button or using the keyboard shortcut O for "one." 

Courtesy Jason Hill Photography

Switching photos

When viewing photos in 2-Up mode, you can only select one active photo pane at a time. Switch images in the active pane by using the left and right arrow keys or by selecting a different photo from the filmstrip preview. The photo on the other pane does not change and serves as a reference photo for comparison.  Use the keyboard shortcut G to copy a photo from your active pane into your reference pane.  (This shortcut was named after Greg Gorman, the photographer who requested the feature.)