Multiple ego vehicles are not supported by the Driving Scenario Designer App currently.
There are potential workarounds for some special cases. If you want only one ego vehicle on the road at any point in time, one possible solution is to have the ego vehicle travel from start point to end point and then to set its next waypoint back to the starting point. You would also in this case set the velocity from the end point sufficiently high to trick the sampling time of the trajectory calculation to avoid detecting any objects while moving/resetting from the end point.
If, on the other hand, your use case is to have one ego vehicle start and travel its path by some distance and in the meantime start the next ego vehicle from the original starting point, that is not possible. As I said earlier, concurrent ego vehicles are not yet supported by the App.
