Hello @Kitanotori,
Secrets are namespaced for isolation, and to allow RBAC control over who has access to what secrets.
Core Kubernetes objects only allow access to secrets that exist in the same namespace as the object itself: Pods, Volumes, etc.
Allowing users to reference secrets in other namespaces can lead to secret fishing, and have security implications.
The fact that you are using the same content (your certificate) across multiple isolated namespaces may suggest some design improvements to your architecture.
In keeping with the patterning and spirit of the kubernetes ecosystem, it is unlikely that cross-namespace secret mounting will be implemented.