Well, you have some controls:
You can designate a friend and assign them the friend slot.
You can increase your slot allocation so that you have fewer upload slots with greater bandwidth.
You can configure aMule to try to upload complete chunks to clients. (This is the default, I think.) Since a chunk is the unit which clients can then start sharing among themselves, this is similar to your idea but at a finer granularity.
However, there are problems with your suggested scheme. What if the person selected as the one to receive the file at high speeds can't actually receive the file at high speeds? What if they can receive it, but can't or don't upload at high speeds? What if they get the file and then stop sharing it? What if they have a feature in their client so that they only upload it to one person at a time, as you want to do? Etc.
The most efficient system is to give each client different chunks of the file so that they can begin sharing them among themselves. Luckily, this is what aMule already does, or tries to do as best it can.
