The complex ac susceptibility χ′ - jχ′′ of a pair of distant square superconducting twin films is calculated based on the critical-state model with a constant critical current density Jc. The results are compared with the measured ones of several pairs of square superconducting films. The theoretical and experimental results agree qualitatively with interesting quantitative differences. It is found that the product of the low-field χ ′ and the field amplitude Hm at maximum χ′′ is insensitive to the film distance, so that the formula for Jc determination of a single film from ac susceptibility measurements may be used for twin films. © 2011 IOP Publishing Ltd.