50g

I solved the problem by moving SIMP after NEXT :-) :

\<< \-> F V N
  \<< F XQ 1 N
    FOR K V DERIV
    NEXT
  \>> SIMPLIFY
\>> 'STO' DERVN

'F(X,Y)=X*Y/(X^2+Y^2)' 'Y' 2 --> 'd2d2F(X,Y)=-((6*Y*X^3-2*Y^3*X)/
(X^6+3*Y^2*X^4+3*Y^4*X^2+Y^6))'