yes, a given observation field and a given observation will have a 1:1 relationship. this is not to say, though, that you couldn’t make multiple observation fields (as cthawley suggested), or add multiple values in a single field (if that field will allow it, such as adding a comma-separated list of values in a open text field).
there’s been discussion of allowing multiple values to be input in a field, or multiple instances of the same field, but those requests have been rejected. for example: https://forum.inaturalist.org/t/allow-multiple-taxa-as-value-of-a-single-observation-field/19075.