Can you simultaneously back up a photo to iCloud Photos and Google Photos on iPhone when you shoot it?

It would be easy to test this. Just set both services up to backup, then log in on a computer and see if your large images are in both places.

The Google Photos app does not start uploading new images until you open the app. If you are concerned about timing, make sure to open the app before shooting the images, which should get you the quickest response.

Note that Google Photos will not necessarily upload your original. Anything over 16MP gets resized down to 16MP. (See https://www.google.com/photos/about/) I'm not sure if they have a paid subscription that removes this restriction. I believe they apply jpeg compression as well. Test it out to see if you are satisfied with the results before considering this a real backup.

Yes, you can do this. I know this because I have done this.

Initially I backed up only to iCloud, but since the iCloud appear to stop synchronizing after a while to my Windows PC, I added also sycnhronizing to Google Photos. The latter was reliable, although I abandoned that too when my account reached the 18 GB limit. (You can upload up to 18 GB of images in their native resolution. If you want more, you'd either have to pay or accept that Google will reduce your images to 16 megapixels.)