I believe that downstream and upstream traffic are separate (merely going from the fact that providers list connections by saying things like 'Maximum 1.54M downstream and 1.54M upstream bandwidth')
No you would not be able to 'link' the two channels for a combined speed (if this is possible I'd be amazed since the upstream channel is so underutilised in many connections)
The speed of the connection is defined by many parameters... too many to list, but hardware limitations of modems and other terminating equipment, type and speed of the cabling/connection medium, amount of traffic present on the segment... etc etc
Identifying all the factors would be very difficult.
There are various types of DSL: ADSL (which most people have), SDSL, IDSL (ISDN really). You have no control over the speed as this is by your DSL provider.
They will cap the speed to whatever speed you have paid for. Typically, in ADSL, they set your down load speed to a higher speed than your upload with the idea that most people need a bigger pipe to download graphics, video and sound.
Most don't need as fast a connection to upload as most people are just sending small requests. If you look at your web pages (view source), you will see that it can be quite large. Multiply that by all the multimedia on the page. When you send a request, it is in essence what you see in your URL Address box. If you are submitting a request, such as posting this message, it will only be what is necessary (the whole page doesn't get sent back.
If you want to pay a little more and need a faster upload you use SDSL (if available), which will be the same speed up and down (still 2 channels, however - and when I say channels, I am really saying pipes). If you are connecting 2 offices and need to send and receive lots of data, you may want this.
IDSL - is just ISDN and is mainly for areas that are two far for normal DSL.