Can Photos compare 2 photos side by side?

No, Photos is basically an app to store your photos, it's not as powerful as other apps such as Aperture or Lightroom...

How to compare two or more photos side to side with Photo for OS-X

Not side-by-side, but you can toggle between the adjusted version and the original master file in Edit mode by pressing the "M" key. While you are holding down M you will see the original, when you release M it will switch back to the original. It is very easy to compare this way.

If you alway want to see the original and the edited version side-by-side, drag one of them to the desktop and double-click it to open it in Preview, while you are viewing the other version in Photos.

Thanks for that. Toggling is better than nothing, but I still think side by side is better and that has now gone, which I believe is a backward step.IMO

Yes, it is. 😟

It would not be so bad, if the zoom slider had a wider range, so we could at least enlarge the thumbnails to a decent size.

As it is, it is nearly impossible to compare two different photos and to decide which of them to keep and which to delete.

One more thing - consider to invest into PowerPhotos as a companian to Photos.

PowerPhotos lets you browse your Photos Library or multiple Photos Libraries in a very versatile way. You can display titles, caption, the filename, or the keywords below the thumbnails or in list view of all photos in an album.

You can enlarge the thumbnails, so you can compare two photos side-by-side. And if you want to merge Photos Libraries, you can easily move photos between libraries. For example, comparing two photos in PowerPhotos.

You can download a trial version here:
Fat Cat Software