[quote=@Flamelord]
<Snipped quote by TwilightDragon>

Well, technically speaking two mages can make a contract with a single servant (See Lancer in F/Z). Alternatively, Association representative, or the Church judge that protects defeated Masters.
[/quote]
That isn't two masters being contracted. Kayneth had the seals and the connection but Sola Ui provided the mana as then Kayneth would be at full power always which was an exceptional strategy considering how powerful kayneth is as a sixth generation head and head lecturer.