how to view 2 photos side by side on MacBook air

Photos for Mac has no tool to show two photos side-by-side and to compare them while viewing them enlarged. you can open each of the photos you want to compare with Preview as an external editor, and drag both preview windows next to each other to compare the photos.

Or start a Print dialog with the two photos selected and select a print template with two photos side-by-side, then print them to PDF and view the PDF file.