hht of a signal which is constituted by two different frequency

my signal is below: fs=2.3000e+05; dt=1/fs; time=0:dt:0.0261; x1=sin(2*pi*200.*time); x2=sin(2*pi*500.*time); x3=x1+x2; figure hht(imf,fs);

The imf of x1 and x2 only have one imf. For x3 , it is constituted by x1 and x2, so the imf should be 2, for my understanding. And the hht plot only composed by two straight lines at frequency 200 and 500 Hz, but it is not. Can someone tell me why? Thank you very much.