Quantum-chemical calculations, incorporating both electron-phonon and electron-electron effects, are performed on two-chain model aggregates to assess the influence of interchain interactions on the nature of the singly charged species (polarons) in organic conjugated polymers. In symmetric cofacial dimers with fixed intermolecular distances, our theoretical approach leads to polarons fully delocalized over the two conjugated chains. Such a delocalization strongly affects the geometric and electronic relaxation phenomena induced by charge injection, which in turn leads to a dramatic spectral redistribution of the linear absorption cross section. Optical signatures for delocalized polarons in conjugated materials are identified from our theoretical description and compared to experimental data from optical charge modulation spectroscopy; these are fingerprints for the degree of interchain order, a decisive factor in the achievement of high charge carrier mobilities in polymer-based field-effect transistors.