Yamaha DTXpress/DTXplorer/DTXtreme

After spending some time with the manuals I am unable to use my 2nd 
DTXpressII brain (and kit) with my first DTXpressII brain (and kit).  
I am attempting to hookup via midi, and all the posts I have searched 
state it can be done but not 'how'.  Can someone post a link that 
would explain this or perhaps explain it 'down and dirty'.  I have 
limited experience with MIDI but I can read and comprehend the manual 
and your messages.

Thanks very much
D.