multiple connections are like adding multiple streets leading to your residence. You can't jsut send half of a truck down each road and send twice as many trucks now can you
The sad truth is that unless your isp supports it, you can only send complete data over each line, and each one will have a different IP address and be on a different network.
It is still useful for SOME people though. For example, running a game server on one while you download feverishly on the other etc. You would need a good router that would allow for this using what are refered to as 'routes' and 'metrics.'