Here’s what I usually write: “Welcome to iNaturalist! Your photos show different species. Please post these species separately, so we can give each one its own name. Here are some tips for splitting the observation up here: https://forum.inaturalist.org/t/how-to-fix-your-observation-with-photos-of-multiple-species/15096”
If the 1st photo has been ID’d (a too easy mistake to make, as I know too well), I bump it up to the highest common denominator ID and then add my little copypasta which will alert the earlier identifier as well as the observer.