Yamaha DTXpress/DTXplorer/DTXtreme

I know this question has been asked a lot, but I'm still not clear.  
I know you can add a "Y" cable to channel 9/10 but on 1-8 I read 
where these are stereo inputs and don't act like channel 9/10.  Can 
you add a "Y" cable to these channels and be able to pull two pads 
off of them.  If you could just imagine 18 pads!!!