There are a few options if you want to load balance multiple internet connections, with certain limitations.
If you're load balancing two 6mbit lines, typically you would be able to download two files at 6mbit each, but not one file at 12mbit (unless you're using a download manager or other multi-connection download program). If one internet connection goes down, all outgoing connections will automatically be routed through the other WAN.
My favorite for load balancing connections is
pFSense, which is free. Another good one is
Untangle, but the multi-WAN module for that costs a few bucks a month to use (though Untangle is free).
Both of those solutions traditionally require a dedicated computer running one of the mentioned routing platforms. The actual hardware can be pretty old, but you will need at least three ethernet ports if you want to load balance two WANs, but they can be PCI or whatever.
As for running this off of your computer, it's harder to do that way. The easiest would be to run
Untangle for Windows, which is essentially Untangle and Debian installed in a standalone VMware Player package. I don't think you need VMware player installed to use it.
In my eyes, the best solution for any multi-wan connectivity is probably pFSense installed on dedicated hardware (AKA an old computer), or a commercial-grade multi-wan router (~$100-250 for a decent one).
Hope this helps!