What, exactly, are you wanting to do?
As noted above, 2.1 is really just stereo. Using surround here is a pretty pointless exercise.
However, If you are trying to create a 5.1 surround project, but you only have stereo monitors, you can assign the outputs in the top field. It's a bit tricky because you don't get the full effect until you play the file back on a true surround system. You need to have a good grasp of the spatial position of what you want to represent.
For the most part, stereo is fine for music.
Surround comes into its own with special effects, foley, etc.
So my question is where are you going with your question?