Thank you, JDiver and johan rebel. I couldn't get an answer from any of the safari operators with whom I was exchanging emails. Thank God for knowledgeable and helpful FlyerTalkers!
Since there is only the one border crossing between the two countries, and since I am coming in December when animal sightings are better in Tanzania, should I give up going to the Masai Mara and concentrate on game parks in Tanzania? What about Amboseli? It is on the way from Salt Lick to Tanzania; is it worthwhile?