For adding multiple sites on nginx server you should utilize Nginx Server Blocks (Virtual Hosts). More about Nginx Server Blocks you can read in this DigitalOcean tutorial.

The main idea is to have one VH to point on php-5, while another will point to php-7.
This could be done by setting correct php path under ~.php$ location in Server Block config file.
Taken from Ubuntu 14.04 LEMP tutorial your php config should be something like…

...
    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass unix:/var/run/php5-fpm.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }
...

For php-7 we will reefer to Ubuntu 16.04 LEMP tutorial, here it should look like

...
   location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/run/php/php7.0-fpm.sock;
    }
...

For full guide reefer to links I given you, they’re pretty good-written tutorials. :)
I hope this will fix problem