Co-simulation refers to simulating a complex system using several coupled numerical models. Engineers define the rate of data exchange between the models by setting communication steps. FMI is a standardized interface which easily allows coupling and co-simulation of numerical models. The RCOSIM approach allows the parallelization on multi-core processors of co-simulations using the FMI standard. In this paper, we tackle the limitations of this approach. First, we extend the co-simulation to multi-rate, i.e. with different communication steps. We present graph transformation rules and an algorithm that allow the execution of each model at its respective rate while ensuring correct data exchange between models. Second, we present an acyclic orientation heuristic for handling mutual exclusion constraints between operations that belong to the same model due to the non-thread-safe implementation of FMI. We evaluate the obtained speedup on a multi-core processor and the effect on the accuracy of the numerical results.