If the protection on the primary is less than 125% of the transformer rating, per NEC Table 450.3(B), no secondary protection is required. If the primary OCP is larger than 125%, the secondary protection cannot be more than 125%, and two 100A breakers is in violation.
So... if that transformer is fed from a 60A breaker, I think you're OK. At least from a transformer protection standpoint; ampacity, etc, is a different issue.
