Duo Security vs GreyCastle Security: What are the differences?

Developers describe Duo Security as "Verifies the identity of your users with two-factor authentication". In order to trust that your users are who they say they are, we verify their identity with an easy-to-use two-factor authentication solution, while giving you the ability to enforce stronger user access policies. On the other hand, GreyCastle Security is detailed as "Industry’s leading provider of risk assessment, mitigation, and certification-readiness services". It is a service provider dedicated exclusively to cybersecurity and the practical management of cybersecurity risks.It provides risk assessment, mitigation, and certification-readiness services.

Duo Security and GreyCastle Security can be categorized as "Security" tools.