Accurate representation of myocardial anisotropy and active stress fields is essential for patient-specific predictive digital twins of cardiac mechanics. While full-field displacement measurements from 3D cine DENSE MRI provide rich kinematic information, the spatially resolved identification of myocardial fiber architecture and active stress remains a challenging inverse problem, particularly in patient-specific 3D--0D coupled closed-loop biventricular cardiac models. In this work, we present a first step toward the joint identification of the direction of myocardial fibers and active stress fields from full-field displacement data. The forward problem consists of a 3D nonlinear finite element model of the biventricular myocardium coupled to a calibrated 0D closed-loop circulation model, ensuring patient-specific pressure--volume loading. Active contraction is described using an active stress formulation acting along the local fiber direction, parameterized by a spatially varying fiber angle and active stress magnitude, while passive constitutive parameters are kept fixed. At the present stage, synthetic full-field displacement data are employed for method development and proof of concept. The inverse problem is formulated as the minimization of a strain-based misfit functional between measured and computed systolic configurations. The optimization problem is solved using L-BFGS, and path-dependent adjoint gradients for transient problems, enabling efficient optimization with tens of thousands of spatially distributed parameters. Physiologically motivated total variation regularization enforces smoothness and boundedness of the identified fields. The results demonstrate that systolic full-field kinematics can constrain both anisotropy direction and local contractility. In future work, the proposed framework will be applied to full-field displacement measurements obtained from 3D cine DENSE MRI in a clinical application context.