To overlap multiple samples across the same key, simply drag the keyzones on top of eachother.
If you want to remove keyzones, select the specific sample in the sample list, then the keyzone attached to that specific sample comes to front so you can select and manipulate it.
To have them played individually one at a time, go to the lower left corner of the keyzone editor and choose another option in the “overlap” drop-down (Cycle or random, by default it is set to “Play all”)
The latter however only works in Renoise 3.0!