This is the system I use for choke and edge triggers on an acoustic (muted) cymbal.
http://img172.imageshack.us/my.php?imag ... 669tn4.jpg
http://img375.imageshack.us/my.php?imag ... 670xd0.jpg
It is pretty simple and it works like a charm. YOu only need one piezo (inside the box) and an aluminum bar. At the end of the bar a microswitch is sticked (it is connected to ring and sleeve of jack). Sticked to the cymbal is a piece of rubber. This piece is only for keeping the microswitch "ready" to be pressed with the only force of a hit it the edge of the cymbal. The triggering is wonderful. No matter how hard you hit the bow, the edge is not activated. If you hit the edge then the microswitch is pushed and the edge sound activated. You can also grab the aluminum bar with the hand to perform a choke.
Give it a try. It is easier than having to build a film switch like hellfire or struggling with two impossible-to-isolate piezos.