How do I plan companion planting, like basil between my tomatoes?

Plan the basil as its own planting marked Interplanted, and one bed earns twice without the tomatoes losing their space in your plan. The tomatoes stay a normal planting. Create the basil in the Game Plan with its own cultivar and dates, turn on Interplanted in Planting Details, next to Location type, and pick the same location type as the tomatoes.

Then, on the Crop Map, drop the basil on the tomato bed: it lays over the tomatoes without pushing anything aside, and its seeds, tasks and harvests count like any planting’s. The steps are in Plan an interplanted planting.