If a CA is compromised critically (private key stolen), then it's removed from the set of trusted CAs you can find in browsers or at OS level (ex: ca-certificat

If a CA is compromised critically (private key stolen), then it's removed from the set of trusted CAs you can find in browsers or at OS level (ex: ca-certificates package on Ubuntu/Debian), which in turn makes all the certificates the CA has signed invalid. With multiple signatures, at least, your certificate is still valid as it has at least another valid CA signature. Also, right now, if one CA is compromised and if this goes unnoticed, the effect is a bit catastrophic as it's possible to create valid certificates for any domain. With multiple CA signatures in conjunction with a security policy mechanism stating how many of these signatures must be valid, you mitigate this issue as it would require several CAs to be compromised and controlled by the same actor to issue valid certificates.