Is it possible (on an S90XS) to control expression separately for two parts in a multi, using two foot controllers?

I know that independent control of voice levels can be done, say, in a performance by using FC1 to control expression for one voice and FC2 to control volume on the other. But I would like to get the benefits of using [removed]maintaining proportionality and balance) on both parts. 

Is there a way to do this using zones and/or different MIDI channels in a master or multi? If not that way, could it be done with an external controller via the MIDI or USB connection?

Thank you!