Hi Luke,
Not sure why the outputs of highpass() and lowpass() would be multiplied. If anything, it looks like they should be added
% create some data
Fs = 5000;
rng(100);
y = randn(1024,1);
% DTFT of the input
[hy,wy] = freqz(y,1,2^16);
% the output and its DTFT
notchfilt = highpass(y,1000,Fs) + lowpass(y,100,Fs);
[hn,wn] = freqz(notchfilt,1,2^16);
figure
plot(wy/2/pi*Fs,[abs(hy) , abs(hn)]),grid
Having said, designing the bandstop filter directly is the way to go.
