Yes, if you have multiple internet connections available they can be "bonded" with certain software or hardware to act as a single connection.
Internet Gateway for ViacomSoft (PC and Mac compatible):
»www.vicomsoft.com/vig/vi ··· ain.html
This software is router/firewall/NAT solutions that allow connection teaming, be it 2 or more cable modems, cable/dsl, cable/dial-up, or any other combo. They both allow parallel downloading and/or fail-back connections (where one connection takes over if the other fails), in addition to all the other features offered.
There are also a few different hardware options out there too, that do a similar thing for a high price.
Symantec Symantec Firewall/VPN Appliance (Models 200 or 200R):
»enterprisesecurity.syman ··· 63&EID=0