The concept of creating a patch with splits and layers is missing, or is difficult to configure on a lot of software-based virtual studios. With Ableton Live 6 this is now easy to achieve by using Instrument Racks.
What can you do with this feature? Well you can create a patches that are made up of a combination of instrument sounds that are layered and split across the keyboard. This allows you to play multiple sounds without having to switch patches in mid-performance and is a must for live gigging.
For example, you could assign a bass instrument to the bottom third of the keyboard, a piano/string layer to the middle keyboard, and a synth lead to the top third. You might want to set the middle piano/string layer so that that it uses velocity cross fade so when play lightly only the strings sound, if you play with medium velocity both the piano and the strings sound, and if you play hard only the piano sounds. Another use for layers is to build lush, fat, and or rhythmically complex sounds by layering a lot of instruments over the same keyboard zone.
Lets checkout a presets created by Ableton so you can hear this for yourself.
Go to the Device Browser-> Instruments -> Instrument Rack -> Performance. Then drag Burton Split into a track or onto the clip/device drop area. This Instrument Rack has bells on the top-half of the keyboard, and a metallic/breathy choir sound on the bottom half of the keyboard.
There are a lot of other presets to listen to and experiment with in the device browser - so have it it!