As of Blender 2.81, both noisy and denoised images can be rendered in a single pass using node based compositing.
Warning, the Denoise Node is using Intel OpenImageDenoise library which is different than the original Cycles one that you get with the Denoising Check box in your View Layer setting.
- In the View Layer, turn on denoising data in your render pass
- Create a compositing node with the new Denoise node and a File Output for your denoised render
Notes: I have found that using HDR check box in the denoise node gives more detailed results in highlights with default rendering settings. This may vary depending on your scene or compositing filters. Also, all the
Denoise Data outputs like Depth, Shadowing, Variance etc.. are computed internally but not used by the current Denoising node. See https://developer.blender.org/D4304 for more info.