Currently if 2 samples occupy the same keymap zone with round robin enabled there is no way to layer them.
If there was a way to specify grouping (maybe simply through the existing Mute Groups) then the complexity of deeply sampled instruments could really open up.
For example a snare drum with 4 round robins instances and 2 different microphone positon samples for each could be layered and sent to separate mod lanes allowing the mic positions to be mixed to taste.
Or a 5 round robins of single analog oscillator could be doubled & layered and used to create a randomly drifting/beating synth voice.