Can you use 2 keyboards in the Sunday Keys App?

You can connect 2 keyboards to a USB hub and the Sunday Keys App, however the sounds and on-screen keys would just be controlled by both keyboards, there isn't a way to designate different sounds. 

If you have the Primary MIDI Controller set to respond to All Devices, you could use multiple MIDI keyboards.

This setup would begin to have issues if you wanted to map knobs/faders. If one controller used CC-11 for a fader and the other used CC-11 for a button, then you could not separate those two physical controls from controlling the same on-screen parameters.