Yamaha DTXpress/DTXplorer/DTXtreme

I got a dtxpress iii kit rather than trying to upgrade that dd502 kit.
 And happened to snag a second iii brain cheap.  And happened to
accidently acquire a TP120SD.  Wow I am so stoked by this kit.  8-)

My goals are to plug a bunch of stuff into the extra brain like DIY
triggers and stuff.  I want the whole kit to have midi I/O to my
computer.  I'm not much of a drummer so I'll want the computer playing
out midi to run the kit including the custom stuff on the second brain
while I play guitar.

What's the best thing to do with the now extra TP65S?  I plugged it
into the extra brain's snare port but I can't get it to make a
different sound than what the main brain's snare is set for.

I plugged all the normal kit stuff into brain A.  Plugged audio outs
of brain A into amp and speakers.  I plugged brain B into brain A via
midi.  Computer midi goes into B and out A.  When I change the sound
for the extra snare pad on brain B it just sounds the same, I guess it
is sending the exact same signal as the snare on brain A is, and brain
A is set to reproduce it as X sound?  So I need to get brain A to
think the snare port signal from brain B is something else, right?

What specifically do I need to change to get things to cooperate?  Am
I correct in my assumption that I can send midi from the computer to
brain B which goes into brain A and brain A can produce all the sounds
from either computer or brain B?  I changed the two brains channels
from "all" to 1 and 2 respectively.  I just so barely understand the
implications of this...

What am I gonna want to do to get this setup to work?  I've spent more
time w/ the manual than I care to admit, I just don't understand some
key concepts here evidently.