Does an interplanted crop take space in the bed?

No, so your bed space is never counted twice and the main crop keeps its room. On the Crop Map, an interplanted planting lays over the main crop without pushing anything aside, and it is left out of the weekly Used Space of your Game Plan.

Everything else works like any planting: its seeds, its nursery sowing, its tasks and its harvests. See How interplanting works.