I'm reading the Fate/Stay Night visual novel. Early on, in the Kotomine Church, you can reach a bad ending by choosing to have Shirou give up his Command Spells and withdraw from the Holy Grail War as a Master. He goes outside to sever his contract with Saber, who says that normally she would kill a Master who abandoned her. Shirou asks if she'll form a contract with another Master, and Saber says that Archer's Master (Rin Tousaka) should be able to bring out her full power. Upon parting with Shirou, she goes into the church, presumably to form a contract with Rin.
Also, in the 2006 Studio Deen Fate/Stay Night anime (I haven't reached this far yet in the visual novel, but I assume it's similar),
Kirei Kotomine commands both Zero Archer (Gilgamesh) and Stay Night Lancer (Cu Chulainn). I assume he was officially the Master of at least Lancer, since unlike Gilgamesh, Lancer would have required a contract to stay in the world.
But I can't remember if it was ever stated that he is the official Master of both, or if
Gilgamesh was maintaining his existence with his own mana, and just stuck with Kirei for his own reasons.
We do know that it's possible for a Servant to have two Masters, as, for a time, both
Kayneth and Sola-Ui are Masters to Lancer
in Fate/Zero. But are the scenarios above instances of Masters with two Servants? Would Rin have been able to make a contract with Saber while also maintaining a contract with Archer, or was Saber relying on her knowledge that Rin wanted her as a Servant, and would cut off Archer if Saber became available? Was
Kirei the Master of both Archer and Lancer, or only of Lancer, with Archer a free agent?
If the answer is yes, do we know how the situation with Command Spells works? I assume this isn't a problem for
Kirei, as he gained all the leftover Command Spells from previous wars when he found his father dying from Kayneth's bullet in Fate/Zero.
But would Rin have gotten an extra three Command Spells if she made a contract with Saber, or would she have to save the one she had left as a sign of her control over both Saber and Archer?
I'm still early into the visual novel, and I haven't yet seen the last four episodes of Fate/Zero, so please use spoiler tags as needed.