a dual wan router may be able to perform load balancing (ex. sending an e-mail goes out 1 wan connection, an ichat session goes out the other) so that both lines are utilized, and offer failover in case one of the lines die. but it's important to note that you can't have a 3MB DSL line, and a 10MB cable line, and say that you have a 13MB connection because they cannot operate as if they're one connection.
In contrast, if we were still back in the days of using ISDN connections, and you had 2 64kbps ISDN PRI lines from the same telephone company, you could bond them together to operate at 128kbps. Or bond 23 PRIs to make a T1 connection. Hope this helps.