I not sure of obtaining multiple plots in the same window using fvtool. Instead you can get what you required, using functions freqz(), plot() and hold on.
[z,fs]=audioread('bird1.mp3');
bfil=fft(z); %fft of input signal
wn=[4000 8000]/(fs/2); %bandpass
[b,a]=butter(6,wn);
[h,w]=freqz(b,a);
plot(w/pi,20*log10(abs(h)),'r');
hold on;
wn=[8000 12000]/(fs/2);
[b,a]=butter(6,wn);
[h,w]=freqz(b,a);
plot(w/pi,20*log10(abs(h)),'b');
