Diastereomeric interactions in 2D crystals formed at solid surfaces serve as excellent models for understanding molecular recognition in biomineralization. Adsorption of a pentahelicene racemate on a Au(111) surface leads to 2D conglomerate crystallization, i.e., homochiral mirror domains, as observed by scanning tunneling microscopy. Upon mixing 26% of (M)-heptahelicene into the racemate monolayer, only the (M)pentahelicene enantiomorph is observed. This effect is explained by a preferred heterochiral interaction between the different helicene species, suppressing the formation of the pure (P)-pentahelicene enantiomorph. These results shine new light onto stereoselective molecular recognition mediated by van der Waals forces.