Paul - wow! 16? I've got to get the new version. That is fantastic.
Roberto, there is no need to do a crossfade unless you want. If you do not mark an "X" between the slots, then they just play equally, so it is the same as layering them. With 16 now - you can do any concievable "custom" registration of this great instrument.
On switching between the slots, you don't need to because you can switch to a different cell which has a different layout corresponding to the changes you want. The slots are there for creating single customized voice which in the case of the organ would be a particular registration not already covered in the registration samples.
I am very interested in this because it is such a beautiful job of recording one of the great organs of the world. Certainly one of the most outstanding sample libraries VSL ever released.