For example I have one fastcgi_pass location block and I want to process the return values of a request to use in a second fastcgi_pass, is this possible ?

location ... {
...
fastcgi_pass server1;
# server1 returns some values
# process these values and call another fastcgi_pass server
if (!$valuechecked) { return 404; }
fastcgi_pass server2;
}