how can I show two or more files at the same time in Mac Photos

There is no good tool in Photos to compare photos side-by-side. We have to view a photo enlarged and use the arrow keys to toggle back and forth between different versions to compare them.

The best you can do in Photos to compare photos is to put the photos into an album, then use the zoom slider to enlarge the thumbnails.

Or select the two photos, then enter ⌘P to print them and select a layout with two photos on one sheet, for example:

Use just the print preview to compare the photos, you do not have to print them.

Aperture has been a professional application, sadly missed. I loved the Light Table to compare photos. Apple does not even try to turn Photos into a professional app; there is no native Apple replacement for Aperture. The target audience for Photos seem to be users who want to be able to organise the photos easily across all devices, iPhone, iPad, Mac, Apple TV; easy to use without having to read a manual. And for that purpose it is perfect. It is a replacement for iPhoto - in some parts even better than iPhoto, only the batch changing is still not as good as it has been in iPhoto, and there is nearly no native support for projects.