It is not straightforward but it is definitely possible:
d1 = rand(10,1); % Create Data
d2 = rand(20,1)+1; % Create Data
figure(1)
plot(d1) % First Plot
ax1 = gca;
figure(2)
plot(d2) % Second Plot
ax2 = gca;
f3 = figure(3); % Copy Axes To Subplots
ax1_copy = copyobj(ax1,f3);
subplot(2,1,1,ax1_copy)
ax2_copy = copyobj(ax2,f3);
subplot(2,1,2,ax2_copy)
Run this to see that it works, then make the appropriate changes to your code to plot both histograms in the subplots.
