Ok thanks for the video. 
Now it is clear to me (i think) what you were trying to convey.
That video was actually really helpful at illustrating what you want.
Keygroups: could also be considered multi-timbrality or another possible way to achieve the same thing would be a multisample-instrument.
If the goal is to play multiple different “sounds” at the same time on the keybed/pads, then it’s usually considered multi-timbrality. Which is exactly what the wish i posted earlier wanted:
If you want an a single instrument that could contain multiple different sounds that don’t overlap or play at the same time, this could be something a multi-sample instrument could solve. There was a wish for this as well, which sadly was declined also:
Which brings us to Layering: the ability to have a single instrument that plays two samples/sounds at the same time at the press of a button.
This you can already achieve by just resampling (as you said) two samples into a new sample. So placing two samples in two tracks and rendering them to a new sample.
I’m not sure what good a new playback mode for this would do? Unless you are thinking of specific features that are not available right now when just resampling?