Don't think two slaves can unlock the chain. With my solution the vizier's lock goes through rings 3,4,5,6,7 so effectively the links between those rings cannot be broken by two slaves.

So if two slaves were to break the chain, it would have to be between rings 1 and 2, 2 and 3, 7 and 8, or 8 and 9.

If slaves A & B work together; lock C secures rings 1 and 2, lock D secures rings 2 and 3, and locks C and D secure rings 7 and 8 and rings 8 and 9.

If slaves A & C work together; lock B secures rings 1 and 2, locks B and D secure rings 2 and 3, lock D secures rings 7 and 8, and locks B and D secure rings 8 and 9.

If slaves A & D work together; locks B and C secure rings 1 and 2, lock B secures rings 2 and 3, lock C secures rings 7 and 8, and locks B & C secure rings 8 and 9.

If slaves B & C work together; lock A secures rings 1 and 2, locks A and D secure rings 2 and 3 and rings 7 and 8, and lock D secures rings 8 and 9.

If slaves B & D work together; locks A and C secure rings 1 and 2, lock A secures rings 2 and 3, locks A and C secure rings 7 and 8, and lock C secures rings 8 and 9.

If slaves C & D work together; locks A and B secure rings 1 and 2 and rings 2 and 3, lock A secures rings 7 and 8 and lock B secures rings 8 and 9.

A third slaves cooperation is needed in every case.

To break the chain between rings 1 and 2, slaves A,B,C must work together

To break the chain between rings 2 and 3, slaves A,B,D must work together

To break the chain between rings 7 and 8, slaves A,C,D must work together

To break the chain between rings 8 and 9, slaves B,C,D must work together