Play until to the point where you meet Fawkes. After he is freed and gives you the G.E.C.K, leave and do the Raven Rock quest. When you see Fawkes outside, tell him to be your follower then proceed to the Citadel and talk to Star Paladin Cross. When she says "I have something to ask you", respond with the second answer. She will say you have a wry wit like your father and ask to join you. Say that you would be honored and she will automatically join you. You now have both Fawkes and Star Paladin Cross as your followers.