easiest way is to get another machine (windows 2000 server with 2 NICs) to act as a 'network multiplexor' (to bridge the two cable connections together to act like one internet connection). You can get hardware to do the same thing, but it's probably going to cost a lot more (well... I guess you could install 2 Linksys cable routers ($100-$150 each) set them up so their LAN ports are 192.168.1.1 and 192.168.1.2 (one per device of course). On the PC side, you can set up each PC to connect to either Linksys LAN port (gateway) depending on which connection you wanted to go through. You can't combine the connections this way, and it makes setup a bit harder (must be static, must remember the two different gateways), but it's do-able).
I'd just save the cost of one cable line, get a linksys cable router and call it a day...