The correlated Pariser-Parr-Pople model Hamiltonian for interacting pi-electrons is employed for calculating the two-photon absorption (TPA) cross section in trans-stilbene and its derivatives using a correction vector (CV) approach. The resulting TPA cross sections are model exact, since the CV approach is equivalent to a full configuration interaction calculation, and all excited states are included in the sum-over-states for the computation of the two-photon transition matrix element. Our study reproduces the experimental TPA cross section of trans-stilbene satisfactorily. The TPA cross section of 7,8-disubstituted stilbene is computed for the identical nature of both substituents. We find that the TPA cross section depends only weakly on the strength of the electron-donating or -withdrawing character of the substituents. Both electron-donating and electron-withdrawing characters influence the TPA cross section identically. We find that steric changes in the molecule due to pedallike rotation reduces the TPA cross section both with and without substituents. However, twist about the central double bond of the substituted stilbene molecule leads to a large TPA enhancement for twist angle of about 60degrees. Thus, stereochemistry changes, besides electronic effects, are important in enhancing the TPA cross sections.