Yamaha DTXpress/DTXplorer/DTXtreme

Hi

can someone explain why when i attach a 3 zone pad using a stereo cable 
to input 3 (tom) and then assign it as a 2 zone pad in the menus i get 
the 2 sounds - pad and rim  as i would expect

why is it then if i attach 2 mono pads via a stereo splitter  to the 
same input, still calling it a 2 zone pad in the menus 1 only get 1 
sounds from 1 pad ie its only recognising 1 trigger - surely the 2 pads 
should act like rim and main pad of a 2 zone pad?? 

isnt it just a case of 2 triggers into a stereo input - why is it 
making a difference whether its a 2 zone pad or x2 mono pads??? - help 
please