Yamaha DTXpress/DTXplorer/DTXtreme

Hello, everyone.  I hope all is okay with everyone, it's been a while 
since I've posted, although I have trolled in the background 
everyday.  I'm getting ready to move to the next step and purchase a 
second DTX2 and combine the two seperate brains together.  I'm a 
little curious as to the best way to accomplish this task.  Should I 
do a straight MIDI connection between the two, maybe use a mixer to 
do this, or maybe a combo of the both?  I'm not really sure.  I'm not 
currently playing in any live situations, so that is not a factor to 
consider.  Any, and all help, is always appreciated.  Thanks in 
advance...

Brett