Only idea I can get is using another location alongside alias directive for second disk.
With alias directive you can define replacement location for content.

Look at this example config

root /usr/share/nginx/html;
index index.html index.htm;
server_name localhost;

    location / {
        try_files $uri $uri/ =404;
    }

    location /downloads2 {
        alias /mnt/disk1;
    }
}

With this approach you will have following situation:
If you access via http://ip_address/downloads, you will see only content from first disk /
If you access via http://ip_address/downloads2, you will see only content from second disk /mtn/disk1.

If you can use multiple locations, e.g. downloads and downloads2, this will get your job done. Problem is if you want to serve files from both disk on one location - /. You will have hard time with this.