One possibility is to look through your keyboard's tone list and use the ones that already mix two sounds-- these are usually "classic" or common combinations such as "piano and strings" or "electric piano and pad." On the WK-7600 the following tones appear to be combinations of two sounds:
Piano 016 - 020, 035
E. Piano 028 - 036, 061
Organ 040
Guitar/Bass 042 - 043
Strings 019 - 020, 042
Brass 015, 031 - 032
etc.
I didn't try to list all of them, and I omitted ones that double the same instrument (e.g., "octave piano") or that don't specify what instrument tones they combine (e.g., "brass section").
Anyway, you could try including one or more of those tones in a layer-- e.g., "E.Piano Pad 1" (E.Piano 034) layered with "Dist.Gt & Bass" (Guitar/Bass 042).
Other than that, you can send your keyboard's MIDI to a DAW and have the DAW send the notes back on additional MIDI channels to create additional tone layers-- but keep in mind that doing so will eat into your keyboard's polyphony.
