2 internet links

we have 2 inet providers, with 2 different ip addr in one server. 1st ip is the default, default route goes thru it, second have no routes. how to set up squid to use 2 links? option squid.conf:tcp_outgoing_address works fine, but tcpdump -i $eth_second_provider sometimes shows icmp: host 81.211.104.10 unreachable - admin prohibited filter second provider iface should be open, iptables rules are up ... please help, or point to manual.