I always have trouble trying to capture double rainbows! The second rainbow I tend to find can be hard to grab with cameras, since the exposures and saturations for the second fainter rainbow can vary quite a bit from the closer foreground image settings. I suppose shooting HDR might help with that, but in the decades I've been chasing rainbows I can't remember switching to HDR in the heat of the moment. I supect that the computational photography of the iPhone 12 is doing just that, as I think about it. That second bow can be very subtle in images. The sky is darker that inside the bowe also, as in your image.
I was hoping you had some new secrets to share about capturing double rainbow images with an iPhone. 😃
As you point out, one secret of great rainbow images is a good foreground subject.
With that thought in mind, over the years, I have captured rainbows with puffins, orcas, wildebeests, zebras, highland cattle in the Scottish Highlands, whale breath, accacia trees, around my own neighborhood, and behind icebergs in the Arctic, I even have a few moonbows as well. Cumberland River Falls is famous for moonbows.
Still looking for a double as nice as yours though.
Speaking of iPhones as cameras, I just had two 20 x 24 monochromatic metal prints made of an image I shot with an iPhone X; image shot last January. I think the image quality is very good, much better than 35mm film I used to print in a darkroom in past epochs.