Currently, SigmaStudio does not contain a built-in crossfader function. Here is an example of how to implement a basic Left/Right crossfader similar to those found in automotive audio systems.
The test tone is fed into two Software Slew Volume controls. The volume controls are controlled by the two lookup tables (one for left, one for right), and these are controlled by a common index value. The default is set to 16. Values of 0 to 15 will pan the signal to the left, and values of 17 to 32 will pan the signal to the right. An index of 0 represents a "hard left" pan, and an index of 32 represents a "hard right" pan. This index can be updated by the microcontroller (or GPIO if desired) to give the user a single-control crossfader.
The project file, index value derivation, and index value files are attached.