I use the observation field “Associated observation” to add the URL of the other species - I try to make observations of both species when I can. Then I use one of the following observation fields to add the taxon, which has to be manually updated as and when the other taxon gets more finely identified:
Interaction->Herbivore of:
Interaction->Visited flower of:
…
etc.
For example: https://www.inaturalist.org/observations/20288388