What you want is the Linux Advanced Routing and Traffic Control HOWTO. Specifically, the section on Netfilter & iproute - marking packets is a good place to start for this sort of thing.
You'd set up your default route to go over eth0, then in
/etc/iproute2/rt_tables add an entry, say at 200, for "viaeth2". Then you need to set up routing in that table:
ip route add 192.168.70.0/24 dev eth2 src 192.168.70.155 table viaeth2
ip rule add fwmark 1 table viaeth2
Then you can mark packets via iptables that should be routed via that table:
iptables -A PREROUTING -d 192.168.70.152 -t mangle -p tcp --dport 23 \
-j MARK --set-mark 1
This will do what you specifically asked in the original question. If you want to do more general load-balancing across the two interfaces, you want to do "bonding" via the "bond" driver.