Theres possibly a few ways to do this and I've tried neither but :
mupen itself has the two icons to either launch or adjust config.
In the config one theirs an 'input config' option, a 'ppsp_input.conf' file will open with mousepad when you select it with the stylus, as the nubs change to joysticks when you launch the config GUI of this pnd.
In that file, it maybe possible to edit N64 analog = Left analog and then also edit all the right analog entries to swap them over,but you'll need to backup the original contents first and this method sounds laborious and probably futile.
thanks for the reply, I tried using the config option, without much luck. I’ll try the other option later.
This device has such great controls its too bad they are so difficult to incorporate.