Separating upload & download [Linux]

Hi,

Currently, I have cable modem & dial up
connections to the Internet and I was thinking of setting up a Linux as a
gateway server in my home network.

But, the issue is .. is it feasible to set
up so whenever I want to download something from the net, I want the server to use the cable connection. But whenever I want to upload anything through my FTP server, I want to set it up so it uses the dial-up connection.

Anyone can give some clues / comments about
this ? Thanks

Fer