server {
        listen 0.0.0.0:443;
        listen [::]:443;
        server_name  www.example.com;

        ...

        # RSA
        ssl_certificate /etc/nginx/tls/service.rsa.crt;
        ssl_certificate_key /etc/nginx/tls/service.rsa.key;

        # ECDSA
        ssl_certificate /etc/nginx/tls/service.ecdsa.crt;
        ssl_certificate_key /etc/nginx/tls/service.ecdsa.key;

        ...
}