Setting up 2 internet lines for F/T

I need to setup 2 different internet lines for fault tolerance. Here are some info;

Dedicated T1 – Natting done on Firewall Side with a permanent VPN tunnel to other branches;

Cable Internet for Business – Natting done on router(has built-in firewall) side;

Is it possible for me to setup these 2 lines to be fault tolerant? I mean if the T1 line goes down, client pc’s automatically used the cable internet as the failover and vice versa? If possible, do i need a special device?