Model Composer allows many black boxes to share a common Questa co-simulation session. For example, you can set many black boxes to use the same Questa block. In this case, Model Composer automatically combines all black box HDL components into one shared top-level co-simulation component, which is transparent to you. However, only one Questa simulation license is needed to co-simulate several black boxes in the Simulink® simulation.
You can also co-simulate multiple black boxes with the Vivado simulator. Do this by selecting Vivado Simulator as the option for Simulation mode on each black box.