Is upstream zone shared between upstreams?

Hi,

we are using different pools of FastCGI servers in different upstreams, with the 'least_conn' balancing method.

upstream zone_a {
zone server-pool 64k;

least_conn;

server 10.1.0.4:9001;
server 10.1.0.5:9001;
server 10.1.0.6:9001;
}


upstream zone_b {
zone server-pool;

least_conn;

server 10.1.0.4:9002;
server 10.1.0.5:9002;
server 10.1.0.6:9002;
}

and so on...from 9001 to 9020.

As long as we are using the same zone (server-pool) does the 'least_conn' method counts each connection on all pools when it decides to redirect the connection for each different, event if they listen on different ports?

Let's say that 'upstream zone_a' has a lot of connection on first 2 servers, 'will upstream zone_b' direct new connection to third server in this case or, as they listen on different ports, 'least_conn' doesn't take into account connection on 'upstream zone_a'?

Simple question is: sharing the same 'zone' on same hosts but different listen ports is OK in this case?
OR is there a better way?

Thanks.