I found this to work better.
d=0.02; %distance between images
moon = imread('moon.tif');
s1=subplot(121);
imshow(moon);
s2=subplot(122);
imshow(moon);
set(s1,'position',[d 0 0.5-2*d 1])
set(s2,'position',[0.5+d 0 0.5-2*d 1])