Factor out your callback function into its own file so you can call it and pass in the arguments.
You cannot see functions externally that are subfunctions of another function (such as a GUIDE GUI)
I would not recommend this method of programming in any way though. Having one GUI accessing the handles of another GUI is very dodgy design even though it is made possible by methods like showing hidden handles.
Unfortunately the alternative that I use is too complicated to explain in a manner for someone to understand who has never done Matlab OOP programming though so what I said first should work for you.
On a very high level though if you work with classes underlying your UIs you can pass these class objects from one UI to another (assuming they are handle-derived classes to pass by reference), attach listeners and then each GUI still controls all its own behaviour instead of GUIs dipping into each other and potentially creating all manner of hard to find bugs.
