How can I look at two photos at a time?

You can't.

Your best alternative, is to add them both to a temporary album, then open one from the album and use the arrow keys to switch between them in full screen view.

Create a new album and put your two photos into it. Expand the display to full size with the slider. Take a screen shot of the two side by side photos and view the screen shot in the Preview app on your Mac where you can expand it to full size.

-- Rich

That's not a feature of Photos.