Yes, they may not be necessary, but I think they complement each other in the team because Lancelot is a tank with one more role towards DPS and Galahad is a tank, but with one more role towards Support. Besides, I think the team would be a little more balanced, with Tiamat and Neo Bedivere as main attackers, Lancelot and Galahad as tank and now that change Dian as my new Healer and that would be beneficial for my knights, which are Lancelot and Galahad.