We present a formula to directly obtain the final relative energy spread of a trailing beam at the maximal acceleration distance. The formula works for electron beams in a two-bunch plasma wakefield acceleration stage in the so-called nonlinear bubble regime. It only requires the longitudinal profile of the trailing beam and the longitudinal wakefield within the trailing beam at the beginning of an acceleration. This formula not only works well for drive beams and trailing beams with the same initial energies, but is also available for those beams with different initial energies. We find that the relative energy spread of the trailing beam obtained from the formula is determined by the ratio of the trailing beam’s initial energy to the drive beam’s initial energy rather than the specific value of their initial energies. We perform several computational simulations using the quasi-static particle-in-cell code QuickPIC, and the results agree well with that calculated from the formula.