if you use same multisample two times with no detune (0 cents),
the sound will only be reacher.
If you slight detune same multisample (1 - 3 cents), it will be phasey as you describe.
If you detune more (5-8 cents) you become phase, flange effect.
If you detune same multisample on two positions more, you will have detuned instrument.
Thats phisics...
Try same settings for both sounds (same multisample) - levels, effects, pan...
If you want still richer sound,
put same multisample 1 octave higher, with volume 30-32, you add even harmonics. It is nice
So it should be nothing wrong with the keyboard.
Regards,
Željko
