A semiempirical charge sensitivity analysis in atomic resolution is used to examine the regional softness parameters within two pairs of coordinating atoms in cyclization reactions, in a search for possible softness matching rules to be used as diagnostic tools for predicting regiochemistry trends. The cycloaddition reactions of a few typical 1,3-dipoles with the phosphorus-containing dipolarophiles and substituted ethylenes are examined. Two types of the two-reactant charge response properties of strongly interacting subsystems are tested : the valence state (polarizational, P) softnesses of atoms in the polarized reactants, before the globally isoelectronic charge transfer (CT), and the in situ CT softnesses. The present analysis identifies a simple regional softness matching rule in terms of two-reactant properties, called the maximum complementarity rule, which is shown to provide a very successful predictor of regioselectivity, particularly at the P + CT level of description. It correctly indexes a relative accessibility of alternative transition states in a series of illustrative 1,3-dipolar cycloadditions. The rule is compared with the earlier regional HSAB principle, formulated in terms of quantities of the separated reactants. The physical implications of various regional softness matching principles are discussed, and the two-reactant influences on atomic FF indices are examined in some detail.