Large and heterogeneous flow simulations could benefit from partitioning, where each subdomain is solved by an individual, dedicated solver. In this presentation we will investigate and validate the fluid-fluid module of the preCICE OpenFOAM adapter by coupling two incompressible OpenFOAM fluid solvers via a surface Dirichlet-Neumann coupling approach [2][1]. The results of various simple test cases are compared to monolithic OpenFOAM simulations. By utilizing a special pressure boundary condition, the coupled results show only a small error around the coupling interface. The magnitude of this error is dependent on the velocity gradient, the discretization size as well as the cell orthogonality. Looking into the OpenFOAM source code reveals that higher accuracy is only possible by manipulating the solvers themselves and thereby violating the black-box approach of preCICE. The fluid-fluid module is extended to couple temperature with great accuracy for one validation case. Furthermore, support for multiphase flow solvers is added to the adapter. Lastly, custom inlet-outlet boundary conditions are implemented for pressure and velocity, which switch their behavior dynamically depending on the flow direction. The results shown in this presentation provide a basis for future fluid-fluid coupling applications with preCICE.