App Access to Multi-channel USB Audio Device

Hi, I use a Window App called MobileSheets which displays/manages sheetmusic in PDF form. It allows me to attach a stereo audio file per PDF to play on click or automattically. The App itself does not have a dedicated Audio management system so it simply plays through the assigned Windows audio assignment.

I use a USB multichannel audio interface for live performances - MOTU MicroLite mk2 Hybrid. Windows gives me access to all 16 outputs of the device. Assigning the output isn't difficult for my PC audio. 

Problem: I use both Stereo audio tracks and Split audio tracks which allow me to send Mono music to the Right channel and a Click track to the Left utilized for in-ear monitoring (IEMs).

Is there an app (e.g. like a mixer) I can use to go between the two to send to two different stereo pairs?

If it falls solely on Zubersoft, maker of MobileSheets, is that a complicated code for them to allow access to the Windows interface device options (16 USB outputs??