I’ve created two render layers, one with denoising and one without. They both render separately and I have to choose from the menu which one I want to see and/or save.
How can I see them rendered as one image and save them as one image without cut/paste operation afterwards?
You can render them in separate EXR using passes and in compositor you can view them both to compare
I’m sorry, but this is too technical for me 
I know that EXR refers to HDR-image type, but I guess that’s not what you mean here? And I’ve never used compositor in Blender.
I can see the rendered images fine, but not at the same time.
BTW, I’m using 2.79 if that causes some restrictions.
Ok. When you are selecting ‘render image’ you get the viewer showing the render. At that level you can save the image as a png or jpeg or else. The you do a new render, with whatever engine and then save the image as img02… At that level if you do not want to use the compositor to compare the image you can use GIMP and create a frame of the size of the two images and insert them to compare.
Or you can use the compositor. For that you duplicate the scene you are going to render and compare. You render each scene and get two renders with the differences you are seeking… You can also create two separate collections with the same scene and two scene view and then render them separately. In compositor you can then consolidate them by using image and position of the images and joining them using a node math with ‘add’ so that both images are sent to the viewer…
Two other things to note:
I tried version 2.82 which I had downloaded ages ago and forgotten since the UI is too weird after version 2.79. Now I noticed that I still need time to learn the UI alone, not to mention other changes.
For time being I think I’ll stick to the cut and paste routine, unless I find a reasonable compromise between rendering speed and denoising/sample settings.
Reason for having 2 render layers is that I have an image plane in the scene which is blurred beyond recognition when denoised.
OpenEXR is a digital file format created by Industrial Light & Magic to serve as an intermediate data file which would exactly capture the data produced by a render step. No encoding, no loss, no rounding.
MultiLayer OpenEXR is an extension to that idea which was pioneered by The Blender Foundation(!), then quickly adopted by the industry because it puts many “named layers” of data into a single file.
EXR files are the best choice for all of the files that you produce during your render job, including the final. Then, you use that as a data-source for generating the “deliverable” video file(s) that the audience will actually see. In this way, you have never “lost” nor “encoded” anything … except as a necessary step in producing a “video file” which of course is “the end of the line.”
The compositor, like most other things in Blender by now, is based on “nodes,” and it’s actually relatively easy to get familiar with.