One way is to use outward firewall rules, separating different services to either cable.
You can use source addresses, MAC addresses, port numbers, protocols, time of day etc to select from.
Here is a tutorial
to start from.
The netfilter site
also has quite a few "patch-o-matic" choices for pseudo load balancing using iptables rules and kernel patches,
such as outward connections alternating between each of the cables.