I think your approach with the 'multiobjective' function is wrong.
The input 'FitnessFunction' should be returning multiple values - the values of each objective function - aggregated as a vector.
Therefore, in your example, 'multiobjective' should calculate the values of 'f1' and 'f2' from their inputs and return a vector of the calculated values.
Add the line
to the bottom of the 'multiobjective' function.
Further, the function should accept the variables 'x' and 'y' as inputs, not 'f1' and 'f2'.
Hope it Helps!