A T1 is actually slower than most cable modem connections, and many DSL connections. The main reason that people still use them is that it's a guaranteed 1.54 Mbps 24/7 point to point. Cable and DSL have faster peak speeds, but you have to go through the Internet to use them.
Back to the question at hand, even if you had two cable modems with two separate cable lines and two separate monthly bills, you probably wouldn't see an improvement with load balancing across NICs or with a load-balancing router. Why? Because cable modems share bandwidth amongst users on the neighborhood network, and both of your connections would be hooked up to the same neighborhood.
OK, so why not go with cable and DSL? OK, let's say you have a 1.5 Mbps cable line and a 1.5 Mbps DSL line, and let's assume that you have them load balanced somehow. You would have a total of 3.0 Mbps of aggregate bandwidth, but you would never be able to use more than 1.5 Mbps of bandwidth for a single download. Why? Well, the load balancing mechanism can distribute outbound traffic between both circuits without any problems, but the return traffic (the downloads) would always come back in on the circuit where the outbound traffic originated. So if the FTP request goes out your cable line, the FTP data comes back down the cable line, and that line is limited to 1.5 Mbps (in this example). You could download two different files at 1.5 Mbps (theoretical of course) at the same time, but you would never go over 1.5 Mbps.
You're better off just paying for a faster connection.
