Two-dimensional van der Waals (vdW) materials have a full set of highly confined polariton modes, such as low-loss phonon polaritons and dynamically tunable graphene plasmons, which provide a solution for integrated nanophotonic devices by combining the unique advantages of different polaritons. Highly efficient coupling between these complementary polaritons is key to realize the nanoscale optical integration. However, fluctuations of permittivity or geometry at the abrupt interfaces have been demonstrated as perturbations or scatters of polaritons. Here, in-plane plasmon-phonon polariton coupling in an in-plane graphene/hexagonal boron nitride (BN) heterostructure is studied using a full-wave electromagnetic numerical model. Transmittance between different polaritons is proportional to momentum matching, which can be tuned using the graphene Fermi energy. The transmittance between a graphene plasmon and a BN phonon polariton can be controlled between 0% and 100% within the upper Reststrahlen band of the BN. This is central to many photon devices, such as waveguides, wavefront shapers, filters, modulators and switches. Moreover, we simulate near-field interference patterns in an in-plane heterostructure based on the theoretical dispersion relation of polaritons, enabling scattering scanning near-field optical microscopy a potential experimental method to investigate the coupling between different polaritons. This study provides a theoretical basis for efficient coupling of propagation and modulation between different polaritons in in-plane heterostructures of vdW materials, which could pave a way to design nanoscale multi-functional waveguide devices in integrated photonic systems.