Two domains, two SSL certificates, one IP

If you are referring to other SSL settings than certificate / key, I would advice you to move their configuration to the http level in nginx configuration.

This way the settings will be inherited to every server block.

For example, in Debian and derivatives you can create /etc/nginx/conf.d/ssl.conf file, where you put these lines:

ssl_session_cache    shared:SSL:10m;
ssl_session_timeout  10m;

# Perfect Forward Security
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_prefer_server_ciphers on;
ssl_ciphers "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+aRSA+RC4 EECDH EDH+aRSA RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS +RC4 RC4";

The files inside conf.d are included in the http level.

default_server approach doesn't work here.

After this, the server blocks would look like this:

server {
    listen 443 default_server ssl;
    server_name _;

    ssl_certificate /path/to/certificate;
    ssl_certificate_key /path/to/key;
}

And, if you want to override some settings for some domains, then you can re-enter the directives in the server level.