In a previous paper we found that the cofacial intermolecular π-π orbital interaction in stacking dimers significantly changes the longitudinal second hyperpolarizability (γ) of the isolated monomer. On the basis of this result, we investigate the longitudinal γ values of π-conjugated main chains (CnHn+2, 6 ≤ n≤ 16) interacting in both-end regions with two small-size cationic perturbing π-conjugated molecules, that is, allyl cations (C3H3 +). These interacting model systems exhibit remarkable enhancement of γ values as compared with those of isolated main chains in the whole chain-length region. The γ density analysis reveals that this enhancement is described by the virtual charge transfer between both-end perturbing molecules via the main chain. The analysis of orbital correlation diagram between the perturbing molecules and main chain molecule also clarifies that such feature of γ density distribution originates in the "weak intermolecular antibondinglike coupling" between the (lowest unoccupied molecular orbital [LUMO], LUMO+1) of cationic perturbing molecules and (highest unoccupied molecular orbital [HOMO], HOMO-1) of main chain molecule. The current result suggests the possibility of novel nano-size control of nonlinear optical (NLO) properties by adjusting the intermolecular orbital interactions between the main molecule and perturbing molecules. A possible control scheme of longitudinal γ for novel intermolecular interacting NLO systems using modified DNA wires is also proposed.