Stability has been an elusive issue for high-order time integration of two fluids coupled across an interface. Iteration between fluid domains can be used to enforce stability, but then there can be time step restrictions for the iterations to converge. The design of methods is complicated by additional properties like conservation of fluxes between fluids and multirate time stepping that are needed for applications. We propose and investigate an iterative approach that has no time step restriction to achieve a stable, multirate, flux-conservative and high-order accurate method for the fluid–fluid problem. Computational examples also illustrate a clear advantage for computing with large time steps, compared to another recent method.