cache_peer ip.of.server1 parent 80 0 no-query originserver name=server_1
acl sites_server_1 dstdomain www.example.com example.com
cache_peer_access server_1 allow sites_server_1

cache_peer ip.of.server2 parent 80 0 no-query originserver name=server_2
acl sites_server_2 dstdomain www.example.net download.example.com .example.net
cache_peer_access server_2 allow sites_server_2

acl foo urlpath_regex ^/foo

cache_peer ip.of.server1 parent 80 0 no-query originserver name=server1
cache_peer_access server1 deny foo

cache_peer ip.of.server2 parent 80 0 no-query originserver name=server2
cache_peer_access server2 allow foo
cache_peer_access server2 deny all
SquidFaq/SquidAcl - Squid Web Proxy Wiki (squid-cache.org)