Hi Erick,
Some guesses: You are trying to trigger the file scenario through the proxy scenario.
So when the proxy is executed, you want to trigger another interface.
You can try like below:
Suppose your file is created in location \interface\PI\out
You configure the sender adapter in your second scenario to pick the file from the above location with a trigger file. Go through Q4 in the link below:
http://wiki.sdn.sap.com/wiki/display/XI/Sender+File+Adapter+Frequently+Asked+Questions
You can create the trigger file on this location through the proxy interface.
Hope it helps!
Ambrish
PS: Why not ask the user to start and stop the Sender channel "xyz"when he wants to trigger the interface 