Dual-WAN

I am trying to figure out a cheap option for Dual-WAN at my office...

I've read that Merlin has Dual-WAN config available. I have two connections:

Primary: 60/10mbps Cable, dynamic IP thru DHCP
Secondary: 16/0.8mbps DSL, dynamic IP thru PPPoE

I want it to fail over to DSL when Cable craps out (within a minute or two max preferably), and to go back to the primary cable connection when it returns.

Is this feasible/reliable with Merlin firmware on a RT-N66U?

Thanks.