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") to open the Preview window.
From the Preview window, select either horizontal or vertical orientation (keyboard shortcuts H 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."
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.