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.'