FM Frequency division multiplexing

Hi I'm working on a project where I need to read 4 sensors at the same time and record the data hrough the audio jack (single channel). I've tried doing FDM but I'm having some issues with the results. for simplicity,i'm currently assuming that the sensors are 2 audio files and I'm doing the multiplexing and the demultiplexing at the same script (it will be separate stations in the future). I seem to be able to only recover channel 1 and it even appears with the demultiplexed channel 2. I also have some HUGE peaks and I want to know why is that? fianlly, since they are 2 signals, how can I display the sum in frequency domain to check if they were multiplexed correctly? what sampling frequecnies should I use here is my code

if true
  clear all;
BW = 4000;
G=500;
[x , fs]= audioread('chan1_1k_10s.wav');   
Fc= BW*2;         % carrier = 1MHz
FS=2.2*Fc;      % sampling frequency for output signal
deviation=20; % freq. deviation  
SAfm = dsp.SpectrumAnalyzer('SampleRate',FS, ...
  'Title','FM Broadcast Signal');
smod=fmmod(x, Fc, FS, deviation); 
% s1=abs(fft(smod));
% plot(smod);
step(SAfm,smod);
[x2 , fs]= audioread('chan2_1k_10s.wav');   
Fc2= BW*4;           % carrier = 1MHz
FS2=2.2*Fc2;      % sampling frequency for output signal
% deviation=2; % freq. deviation  
SAfm2 = dsp.SpectrumAnalyzer('SampleRate',FS2, ...
  'Title','FM Broadcast Signal');
smod2=fmmod(x2, Fc2, FS2, deviation); 
step(SAfm2,smod2);

sumx = smod + smod2 ;

%% recovery wn1 = [Fc-200 Fc+200]; [A1,B1] = butter(3, wn1/FS/2, 'bandpass'); rec1 = filter(A1, B1, sumx);

%%%%%%%%%%%%%%%%%%%%%%% wn2 = [Fc2-200 Fc2+200]; [A2,B2] = butter(3,wn2/FS2/2, 'bandpass'); rec2 = filter(A2, B2, sumx);

%% demodulation demd1 = fmdemod(rec1,Fc,FS,deviation); [A11,B11] = butter(2, 30/fs/2 , 'low'); dem1 = filter(A11, B11, demd1);

%%%%%%%%%%%%%%%%%%%%%%%%%%% demd2 = fmdemod(rec2,Fc2,FS2,deviation); [A22,B22] = butter(2, 25/fs/2 , 'low'); dem2 = filter(A22, B22, demd2);

%% plotting %channel 1 t = linspace(1/fs,length(x)/fs ,length(x)); figure; subplot(211); plot(t,x);title('Channel 1 before multiplexing'); subplot(212); plot(t, dem1);title('Channel 1 after demultiplexing'); figure; plot(t,x,'c',t,dem1,'b--'); xlabel('Time (s)') ylabel('Amplitude') legend('Original Signal','Demodulated Signal')

%channel 2 figure; subplot(211); plot(t,x2);title('Channel 2 before multiplexing'); subplot(212); plot(t, dem2);title('Channel 2 after demultiplexing'); figure; plot(t,x2,'c',t,dem2,'b--'); xlabel('Time (s)') ylabel('Amplitude') legend('Original Signal2','Demodulated Signal2')

end

It's also attached, I don't know how to display it all. Recovered 1 shows the oiginal 1s file and the recovered ne. recovered2 shows the original ch2 and what looks like thre recovered ch1 again. thank you very much