Obrigado pela ajuda, finalmente encontrei configuração de trabalho (obrigado pessoal da lista de discussão squid)
http_port 3128 name=port_3128
http_port 3127 name=port_3127
nonhierarchical_direct off
acl port_3128_acl myportname port_3128
acl port_3127_acl myportname port_3127
always_direct deny port_3128_acl
always_direct deny port_3127_acl
never_direct allow port_3128_acl
never_direct allow port_3127_acl
# 3128
cache_peer proxy1 parent 3128 0 proxy-only default name=proxy3128
cache_peer_access proxy3128 allow port_3128_acl
cache_peer_access proxy3128 deny all
# 3127
cache_peer proxy2 parent 3128 0 proxy-only default name=proxy3127
cache_peer_access proxy3127 allow port_3127_acl
cache_peer_access proxy3127 deny all