For a federated server-to-server protocol with domain-based identities, I am thinking of using TLS client certificates as authentication mechanism. I figured that re-using each peer's TLS server certificate as client certificate for connecting with other peers would be the simplest approach to implement this.
However, I am not sure if this double usage of a single TLS certificate can be considered secure. Do any special risks arise from using a single certificate as both a server and client certificate, similar to how using a private key for both signing and encrypting is problematic? I want to avoid creating a serious security problem out of mere convenience.