I did a quick Internet search using keywords "Combine DSL Lines", and got quite a few hits. One of those hits took me to an article that states the following:
"The Pro800turbo Load-Balances 2 different Internet connections making them seem as one. This enables full use of BOTH lines at the same time. Users can define percentages of use for each line if they like. Also, if one line goes down, 100% of the traffic is sent to the good line... Automatically! Great for networks with heavy loads or power surfers that like to download multiple files, like MP3s, at once. Speed up your network NOW!"
Link to article:
http://www.nexland.com/turbo.cfm
(REMOVE any SPACES from the pasted URL)
That may be what you're looking for. If not, do a similar search yourself for some different options.