Hi,
You are correct that only 1 BP actor can be possesed at the time. However, if you are attempting to make a homing missile that will track targets without having to maunally guide it. You could simply make your missile a simple AI flying pawn. Then use a nav mesh to give it simple move to commands. Alternatively, you can use a system similar to the one used in the Turned Based Strategy template available free in the market, to use force to move your pawn across the stage without needing a navemesh or even a movement component on it. I can give you more details on either if this is what you are trying to do. If not let me know and I might be able to help you make a mouse guided missile at the cost of abandoning your default pawn while you pilot it.