By extending a previous semi-classical model, we investigate the effects of inserting a second ferromagnetic material at the interface or in the interior of the ferromagnetic layer in NiFe/Cu/NiFe and/or Co/Cu/Co sandwiched structures on their giant magnetoresistance (GMR). The calculated GMR was found to be consistent with experimental results, indicating that our model is applicable even for complicated spin-valve multilayers. Moreover, some theoretical predictions are given in this letter that allow us to propose ways to optimize spin-valve structures. [ABSTRACT FROM AUTHOR]