Why the double tiered setup? Use nginx to serve static content and
then proxy to backend processes (php-fcgi, mongrel or whatever). You
just need different upstreams.

e.g.

upstream site1_mongrels {
#...
}

upstream site2_mongrels {
# ...
}

server {
listen site1.com
#...
proxy_pass site1_mongrels;
}

server {
listen site2.com
# ...
proxy_pass site2_mongrels;
}