Yamaha DTXpress/DTXplorer/DTXtreme

What you describe isn't possible because you need to conver two piezo
signals into a piezo plus FSR signal.  What you need is a "magic box"
which does just that.  If you search the archives you will find many
discussions about it.  

If you can manage a soldering iron there is a circuit in the "files"
section called "cheap DTXpander" which will give you the interface you
need. It is not perfect, although with a bit of tweaking of the
resistor values it can be improved to suit most systems (including
possibly reversing one of the diodes and removing another for Roland
systems).  As it is drawn it worked well on my DTXpress III. 

Keith.