UPD: After a bit of experimenting, it turns out that you have to create a new group for each sample to achieve your objective.
After creating a new group and loading a sample there, just make sure its zone (i.e. notes that will trigger the sample playback) is the same as you assigned to other groups (i.e. samples to be played back simultaneously).

In this case, there are two groups (samples), both assigned to a zone that is limited to C2 note. So, they both will be triggered by the corresponding key.
Apparently, what you're trying to do is layer samples in the same group (obviously, a non-starter, as they cannot be in the same group and occupy the same zone at the same time). To layer samples for simultaneous playback, if I understood your question correctly, you need to create new group for each sample.
Hope it helps clarify this process.