This is for a single proxy. It can be easily modified for multiple IPs. You should be able to do this with something like this.
acl ip1 myip 192.102.51.2
acl ip2 myip 192.102.51.3
tcp_outgoing_address 192.102.51.2 ip1
tcp_outgoing_address 192.102.51.3 ip2
tcp_outgoing_address 192.102.51.1 # default