There is a chinese variant of tomatousb that does support dual wan. For a brief period when I was changing from DSL to Cable, I had both services, so I gave it a shot. It doesn't just simply combine the two services, you're connections have to go down one, or the other, but you can tell how to distribute them. Say your cable is twice as fast as DSL, then you can tell it to send twice as many connections to Cable than DSL. For single connection downloads, you will only be using one service provider. If, however, you're using Bittorrent or Usenet, you can adjust it to use both ISPs, as both those services can establish more than one connection. With Usenet anyway (is what I primarily use) I was able to download at the full combined speed of both services (which was freaking awesome). I had to make some additional QOS rule or something for it to do this, else it will send all connections down just one ISP.
Now for the downsides, the chinese version is closed source. It should be open source, since it's tomato, but what can you do, sue someone in China (lol). So you don't know what you might get. Some people suggest there could be some back door programmed, or other malicious tool into the firmware. I can't say that I noticed anything malicious, but I can't really advocate the use of a closed firmware that should be open. Also, it's very difficult initially to get dualwan going, because it's in Chinese. Once you have the connection up, you can use Chrome to translate the firmware pages.
I would like to see the source for this released, but the Chinese developer hasn't cooperated, and no one else has implemented it.