Yamaha DTXpress/DTXplorer/DTXtreme

Hi Friends,
I am an acoustic Drum player and i am going to buy Yamaha DTXpress 
drum.Hence i have joined this group.I want some details about 
electronic drums.
What is Dual trigger?
What is 2 or 3 Zone pad?
By Midi out connection in the Sound module could i record the Drum 
beats(While playing) in my computer?
How DTXpress111 differs from DTXplorer?
In Roland SPD-11 there is layer system(Layer-A and Layer-B).Both 
layers can be combined i,e When one pad is strucked once it will give 
two sounds at a time(Snare+cymbal or Kick drum+cymbal etc).Like this  
function available in which model?
If anybody could answer me?

Thanking you
Regards,
Mario