The statistical mechanics of two Ising ferromagnetic planes coupled by a local interlayer two-spin interaction have been studied by means of a variety of calculational methods, including different mean-field approximations, Migdal-Kadanoff-type renormalization-group techniques, scaling theory, as well as numerical Monte Carlo simulation techniques. The phase diagram has been derived as a function of the interlayer coupling strength. Furthermore, various thermodynamic variables have been determined, including the interlayer correlation function, which is proportional to the interplanar force. It is found that a Migdal-Kadanoff renormalization with a decimation procedure which involves the interlayer coupling in an appropriate fashion provides an accurate description of the phase diagram and that a mean-field description provides a good description of the phase diagram and the interplanar force, except for very low interlayer coupling strengths.