My first thought would be Linux bonding. However, there are complications because the ISPs will be using different IP space.
A quick search revealed this thread that appears to have some ways folks have attacked and possibly solved the problem.
Although I've done bonding I haven't done the multiple ISP thing. The above thread seems to suggest vtun would be the way to go. I'm not at all familiar with vtun though. Have a look through the link and see if it helps.