How to tell squid use Source IP address for sending requests?
There are many types of acl. src means the client IP and not the interface IP (local address).
I use localip acl type for that purpose:
acl from_eth0 localip A.B.C.D
acl from_eth1 localip E.F.G.H
tcp_outgoing_address A.B.C.D from_eth0
tcp_outgoing_address E.F.G.H from_eth1
tcp_outgoing_address 1.2.3.4 # default
ACL types are described in doc page.
However, it's painful to write each address by hand. I don't think it's a final solution because of that.