Endive
Endive's broad rosettes shade the soil surface, which keeps chervil's roots cool and damp and often delays bolting through late spring. Both crops want the same cool, evenly moist bed, so one watering serves them.
Cilantro
Both are quick cool-season herbs that bolt at similar times, and the small white and pale flower umbrellas they throw feed hoverflies, lacewings, and tiny parasitic wasps that hunt aphids. Let a few plants of each flower at the row end.
Sweet Cicely
Sweet cicely is a tall perennial that casts light dappled shade all summer, the kind of cool footing chervil needs to keep making leaves. Sow chervil on its shaded side rather than under the crown, where roots compete.
Lettuce
Interplanted lettuce and chervil shade the soil between them and hold moisture, which suits two crops that both quit in heat. Harvest cadence matches, so the bed clears at once.
Radish
Radishes swell just below the surface and come out in a month, while chervil sends a slender taproot deeper, so the two rarely fight for water. Pulling radishes also loosens the row without disturbing chervil crowns.