I am not sure if I am understanding the question.
IF, One would have 2 identical apps like TRANSMISSION, operating on the same computer at the same time. Lets assume for this example that this computer is connected to a ultra Wide bandwidth, High speed DSL line and is using TRANSMISSION(x2) with both the apps using the same download folder or download location. . . AND lastly for this example, your downloading any given 2Gb movie file. This arrangement would have some speed gain. It might not be that significant, but you would lesson the load for each torrent client respectively.
I'll try to explain how this works.
Now, you would put the same(identical) or duplicate torrent files into each running TRANSMISSION app. NOW SHUT DOWN one of the 2 apps. As soon as the app you left running actually begins making some progress in building and piecing together your file, stop the 2nd app. (a movie file in this example). NOW, restart the 2nd TRANSMISSION application. It will 1st look into the download location or folder and check to see how much of it, if any is already there. (This is how TRANSMISSION WORKS, I can't speak to other bit torrent clients). After a few moments or a few minutes the 2nd TRANSMISSION APP will KNOW what it already has of the pieces needed to complete the file. -(a 2Gb movie file in this example)-
By default, TRANSMISSION always checks what it has, in fact, it even does this as it goes, all the time, constantly. So if a piece it just found, was already placed in the download folder, it would "see" that it is there already and move on to find another. It would not put 2 identical pieces in. The one app wouldn't know the other app is putting pieces in too, it would just think that piece was already there. No conflict would occur. It doesn't put a file together in any specific order like from beginning of the movie to the end. It might find a piece that is near the end in the 1st 5 minutes of downloading. It will know where to put that piece and will add it without any problem. I do not see how there would be any duplication or corruption as long as the 2 Torrent client were identical apps and would thus deal with finding already existing pieces the same way it does at start up when it looks into a download folder.
If things were started this way at least in the way TRANSMISSION works, you would find a gain in speed.
It would work like this. Lets say for example 2 guys are asked to load 40 items into a single box. The items are placed on a rotating table with one guy on each side. The box (final file or download in this example), is sitting in the exact middle of this rotating table and is easily reached by both guys. Each guy has a digital manifest consisting of a light beside each item and as each item enters the box, BOTH of their manifests have a light that goes out to designate that a particular item has already been added to the box. Note, it is irrelevant which guy put an item in the box, because when an item goes in the box both of the lists/manifests, let each man or each other,(each torrent apps), what has been added and thus what is left to add, and in real time and at the same time.
Even if there were many duplicates of the items needed, having 2 people to do this task would obviously be faster then just one. ONLY if each man was unaware what the other had added, would you end up with duplicates or errors. HOWEVER,in my example, the manifest works just like the way the torrent client checks as it goes, so no error could happen, and you would lesson the load of each app and this would result in a speed gain.