I can't figure out how to split channels into independent files either, and have often wondered if it was possible with WavePad, so I hope an answer is forthcoming. (I use audacity to easily do this type of thing.)
But your inquiry got me searching options in WavePad again, and here is what I've found (so far...)
Under the Effects tab, select Stereo Pan. This will open a new window, with anchor points at each end of the file. I selected one and (for example) dragged it up (left) - and then selected and dragged the other end to make the line level. Not a very exacting way to perform the task, but worth trying.
Surely there is a better way to achieve this and adjust by a percentage as you are looking to do. Marking the thread!