A coupled system of two molecules bearing spins of 1/2, which are allowed to diffuse relative to each other, is considered. By using a symmetry-adapted basis operator set, the overall density matrix equation is decoupled into two equations for the time-resolved isochromat components, the sum of which yields the observed signal. The appropriate stochastic Liouville equation is solved by a combination of eigenfunction expansion and finite-differences for the angular and radial relative motions, respectively. A full range of spectra from classic Pake patterns in the rigid limit to motionally narrowed Lorentzians is recovered. As an extension of the above approach, the solid-echo experiment is described in terms of the ensemble-averaged isochromats. Homogeneous transverse relaxation times (T-2) as a function of the translational diffusion coefficient (D-T) are obtained from simulating SECSY (spin-echo correlation spectroscopy) signals, which show distinct T-2 minima vs D-T. The present method of separating the quantum and stochastic classical variables constitutes a useful approach for treating multiquantum statistical systems, and it can be generalized to an arbitrary number of spins as shown in a companion paper. In the present study we obtained the usual linear dependence of T-2 on D-T in the motional narrowing (or Redfield) limit, whereas in the slow motional regime a D-T(-1/2) dependence is observed, consistent with studies of rotational diffusion. Varying the distance of maximum separation between the two spins (r(max)) has virtually no effect on the location of the T-2 minimum with respect to D-T, implying that the onset of slow motion is essentially independent of r(max).