Bonding an interent connection with 2 or more lines can be quite tricky, althouhg I have done it for a few clients.
I would recomend a Firebrick -
Firebrick - For Firewalls, Bonding ADSL, Routers, Traffic Shaping...
You can use the Firebrick to setup the bonded lines and the firebrick will sort out all your packets a long with ensuring a balanced load.