Edit/compare 2 or more photos on screen?

You can't. It's not a feature of Photos. Tell Apple what missing features you'd like restored or new features added in Photos via https://www.apple.com/feedback/photos.html.

You can view two images side by side if you use PowerPhotos to open one of the images and Photos to open the other and put the two windows side by side.

If you select the two photos and enter ⌘P to open a print dialog, then select a full page print layout, you can see them side-by-side a bit larger than in thumbnail view: