How do I view photos side by side in macOS photos i.e. have two windows open

This is just not a feature of Photos.

For feature requests:

Feedback - Photos - Apple

I usually select the two pictures and use the arrow keys to pop back and forth between them.

Many of us use the trusted 3rd party app PowerPhotos ($40) for the second window.

What I do is put the two photos in an album and move the size slider to as large as possible. If it need it larger for better comparison I have set the System ➙ Zoom Settings to zoom the full screen when I hold down the Control key and swipe up on the Magic Mouse.