centos - How to configure nginx to serve one site from two different document root and using different php depending on URL

I'm trying to configure nginx to use two different document root depending on url, so if some one connects to www.example.com will get site from /portal/public_html but if client connect to www.example.com/account will get sites from other directory /portal2/public and that site should be served by php 7.
I'm using variables in nginx configuration to achieve my goal, but something is not working as I thought it should. I'm getting errors 500. In my logs I see that nginx is looking for site files in his default directory "/usr/share/nginx/html"

2017/11/20 10:31:38 [debug] 21729#0: *421789 fastcgi param: "DOCUMENT_ROOT: /usr/share/nginx/html



Maybe the configuration will be simpler to read and understand then my rusty english so this is my nginx configuration:

server {
    listen 81 default;
    server_name www.example.com;

    access_log /var/log/nginx/www.example.com.access.log;

    error_log /var/log/nginx/www.example.com.error.log debug;

    location / {

    set $my_root  /portal/public_html;

    #PHP v5
        set $php_host_port 127.0.0.1:9000;
        root /$my_root;
        index  index.php;

        proxy_read_timeout 200;

        include /nginx_conf/portal_rewrite_params; 

    }

    location ~^/(account|new-cart|signin|register|n) {
    set $my_root  /portal2/public;
        root   $my_root;
        proxy_read_timeout 200;

        index  index.php;
        try_files $uri $uri/ /index.php$is_args$args;

    #PHP v7  
     set $php_host_port 127.0.0.1:9071;

    }

    location ~ \.php$ {


        fastcgi_pass   $php_host_port;
        fastcgi_index  index.php; 
        include /nginx_conf/fastcgi_params;
        fastcgi_param  SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param  DOCUMENT_ROOT $document_root;
    }
}

So my question how to achieve that nginx serve same site from different document root and using different php depending on URL? Is my approach good ?

My enviroment is centos 7, nginx/1.10.2, php 5.6 and php7.1