question

Don't call FLEXSIMINTERFACE functions from another C++ function.

Add a different function with its own signature and call it from both of those functions.

double Fun1_cpp(double a, double b)
{
  return a+b;
}
 
visible double Fun1(FLEXSIMINTERFACE)
{
  double a = param(1);
  double b = param(2);
  
  return Fun1_cpp(a, b);
}
 
visible double Fun2(FLEXSIMINTERFACE)
{
  double a = param(1);
  double b = param(2);
  double c = Fun1_cpp(a, b);
  
  return c;
}

or

double Fun1(double a, double b)
{
  return a+b;
}
 
visible double Fun1_FS(FLEXSIMINTERFACE)
{
  double a = param(1);
  double b = param(2);
  
  return Fun1(a, b);
}
 
visible double Fun2_FS(FLEXSIMINTERFACE)
{
  double a = param(1);
  double b = param(2);
  double c = Fun1(a, b);
  
  return c;
}