I am sorry I misunderstood you. It looks like you are saying that the source and the receiver are both moving at 10 km/sec. That is a very simple question. It would take exactly 1 second from the source to the receiver and it would take exactly 1 second to return. Both the source and the receiver are in the same inertial frame so they both would agree on the time and distance the light traveled. Consider this. Assume you wake up on what appears to be an airplane but your not sure how or why you are there. Is there any test you can do (other than looking out the window) that will prove that you are either flying at 500 mph or that you are on the ground and you are being tricked into thinking you are flying? In other words are you moving at 0 mph or 500 mph.