I think what you want to do is something like MLPPP to bond multiple lines together. First, I believe your ISP needs to support doing this. Also, this is usually a more advanced move so their isn't much support for this except for businesses. However, I did a quick search and found an interesting video on YouTube. I also found a PDF on how to configure multilink in a few OSs.
I hadn't tried this, but seeing how easy it is makes me want to get another cable connection.
If your ISP doesn't support multilink, I think the next best thing you can do is load balancing, meaning your computer/router would send request on the least busyest line. So you would only use one ISP modem for a connection, but any request could go out whichever connection is free. A problem this could cause is if you login to a website which uses load balancing on their end they may cache the IP you use for your state. In other words, it may appear you were logged out of a website, without warning but that is because you are using your other IP address to connect to the site and they don't realize it is still you. The other thing that may happen are sites like banks may wonder if your session was hijacked. I don't know how likely this is, but I would believe now a day some of these problems have been overcome given how mobile things are getting.
There are some routers/firewalls that can designed for load balancing (ex. Netgear FVX538). I've found a hardware solution is always better but hardware costs money.
I hope these tips help your need for speed. :)