Hi Keith,
You write about 'stereo connectors'. Just to be sure about your setup :
Did you mean 'stereo jacks' (with tip, ring and sleeve)? ...... or use two separate (mono) jacks plugged into a STEREO channel (strip) of the mixer?

If your mixer has both stereo- and mono inputs the suggestion using 2 mono's panned to opposite directions is OK. Technically there is no difference... in fact a stereo strip = 2 mono strips combined. The stereo strip however has 'double-potentiometers' (kind of piggy-back) e.g. adjusting an EQ setting has effect on L and R.
Realize you have to adjust gain, EQ and volume on both mono channels when used for stereo.

If the KN7k or PSR3k has stereo line-inputs you also could use one of them. The soundmodule is mixed with the keyboard.