From my understanding you want to have multiple vehicles as ego vehicle. As of MATLAB R2021a, this is not possible to have multiple ego vehicles. But if you want to controller vehicles just using trajectory then that can be done even for a not ego vehicle. The Driving Scenario Designer App currently supports sensing for a single ego vehicle. Thus, It is neither possible to add sensor controller nor export sensor data for multiple vehicles within the same scenario.
A possible workaround you may consider achieving this same functionality would be to maintain to identical scenarios, with a different vehicle as the ego vehicle in each. This will run the exact same simulation, but with sensor data for different vehicles measured in each scenario. You can then either export each dataset from the App and manipulate them in MATLAB or generate two MATLAB functions for each scenario and produce the datasets that way.