Hadhrat Mirza Ghulam Ahmad is a descendant of Hazrat Fatima (ra)
fayax
you are confusing the issue. Why does this World need to 2 people to save this world? First the Mahdi and then Isa (as). What is the sense of Isa'a (as) coming then?
Why do we have two lungs? Why does the US governement have a Vice President? Why has there been more than two prophets to the same nation at the same time?
Asking why for everything is not our job. Its he same as in asking why does Obama need a several military leaders?
Mainly the explanation given by scholars is that the leader of Muslims, the Mahdi will be a Guided muslim man. But the fitnah of Dajjal will be too strong and only a Propeht (Eisa A.S.) will be able to destroy/kill him.
Remember, Hazrat Eisa was never a political leader, like Hazrat Mohammad (Sallalaho aleihi wassallam) and like Moses (Alehis Salam) and a few others. This time also when Hazrat Eisa will come to earth he will play the part of the religious leader more than the military or the political leader. The military leader will be Mahdi.