If they’re on separate days, it’s best to make a new observation.
Personally I’ll use a comment or edit the description to “link” the observations together, but you can also use an observation field or tags. More details on using an observation field for this kind of thing here: https://forum.inaturalist.org/t/using-the-field-similar-observation-set-for-linking-observations-of-lepidoptera-when-raising-on/1018