well It goes both way and alot of it depends if you are going to use a software or a hardware solution, with software you are correct that you are basicly making a wider pipeline.. but thats only if you are doing thing that use one connection (surfing the web, downloading from a FTP).. in that case you will not get any faster speeds on any one download you are attempting (this also means that using a software solution will not show a faster speed on a internet speed test).. But while using software and download from multiple sources (torrents, newsgrops) will make that one thing you are downloading twice the speed (provided the download can be hosted that fast) because it can max out both connection when trying to download one thing.
on the other hand when you use a hardware solution (Load balancing dual WAN router) you will actually increase the speed of your connection (and it will show on a speed test and when downloaind from a ftp or single source) because the router takes everything and divides it up evenly between the two. A GREAT example is RAID, if you max out one connection with half the data, and max out the other connection with the other half of the data it will get there twice as fast.
I have heard that you get what you pay for when it comes to load balancing routers, the $120 router will not do a very good job (when compared to a $1,000 business grade router). So knowing that to save money alot of people "build" routers out of old pc's using linux and some software. for some reason linux is much better and working with connection that windows, and you can acutally build the equivelant of a very expensive and acurate Load balancing router. The pc must have atleast 3 network interface cards (two to connect the modems, and one to connect to the pc or router), another advantage of using this method is you can add as manny modems as you want just by adding NIC cards.
--BK--