Neither cable, nor DSL will ever play any type of dynamic routing with you - BGP, OSPF or other. You get what you got and that's why it is cheap.
You can do simple outbound load-sharing with two equal-cost default routes and IP CEF enabled. It gets messy when you try to do VPN's and things that require a specific interface/IP to be reached.
Your best bet is simple failover with floating static default route. Your secondary route just gets a higher metric.
You can augment that with SLA's and tracking to have better failover.