Yes. Based on user experience in the replies, iCloud Photos and Google Photos can both be used on the same iPhone without conflicting, so one photo can end up backed up to both services.

A key limitation is that Google Photos on iPhone may not start uploading newly shot images until you open the Google Photos app, so if timing matters, open it before or soon after shooting.

Also, don’t assume Google Photos always stores the exact original file. According to the community answers, images above 16 MP may be reduced to 16 MP unless you use a storage option that preserves full resolution, and JPEG compression may also be applied. So it can work as a second cloud copy, but you should test whether the uploaded files meet your backup-quality needs.

As for iCloud’s optimized storage, the answers don’t give a definite time for how long the original stays on the phone before being replaced by an optimized version. If that detail matters, the safest approach is to test with both services enabled and confirm on a computer that full-size files appear where you expect.