Okay, so you have six physical lines coming in from the street?
You've still only got as much "redundancy" as the lines then (read - not much ... they likely all terminate in the same cabinet down the road). I mean, okay, it's better than one line coming in, and a coax splitter out to six modems, but still not all that great.
Most likely, you'll need to use OSPF in your routers (or maybe BGP would work) in order to properly share out the connections. Though handling the NAT per client will get interesting to say the least -- do they really need individual publics?