sure, you could apply the "rule of mixtures" (factor one sheet thickness by the ratio of Ec) ... but will it make any difference in th elong run ?
the stiffer sheet will react more of the applied load untill the lower bearing allowable stress is reached. the load reacted by this sheet is fixed (Fbr*A), all that would change is the applied load when this is reached (but who cares ?). the load reacted by the stronger sheet is also fixed, so the full load is fixed. the assumption is that the weaker sheet doesn't fail in bearing untill the stronger sheet does, not 100% correct but near enough, no?