Model Exchange vs. Co-Simulation
This depends on how you export your FMUs: You can either use FMI for model-exchange or FMI for co-simulation.
In the model-exchange scenario, the FMU contains only the model and no solver. Therefore the solver of the importing simulator is used.
In the co-simulation scenario, the FMU contains both the model and a solver. Here the importing simulator acts as the master of the co-simulation.
In both cases you will need a simulator that supports the import of the respective FMU type. OpenModelica has support for import of model-exchange FMUs since version 1.8.0. Hence, if you have two FMUs that you have exported using FMI for model-exchange, you can import them both into OpenModelica and simulate them using the OpenModelica solver. Note that strictly speaking, this isn't a real co-simulation since only one solver is involved.
If you want to use a separate solver for each FMU (i.e. perform a real co-simulation) then you should only export one of the models as an FMU using FMI for co-simulation. Import that FMU into the simulator of the other model. Note that OpenModelica 1.8.0 doesn't support FMI for co-simulation.
Model Exchange Example
Here's a working example (tested in Dymola, I don't have OpenModelica installed here right now):
parameter Real p = 1.0;
output Real value;
c.value = p;
input Real value;
result = c.value;
Export both of these as FMUs. Then import them and combine them as follows:
It seems that OpenModelica currently does not support simulating two FMUs simultaneously. This is a known bug.