The error message has nothing to to with using 2 cameras. Your FramesPerTrigger setting is 10. so img1 and img2 will be 4-D matrices, where the fourth dimension is the frames. This is also covered here:
Try following in the FOR loop to see what I meant; obviously, this will only save the first of every block of frames you captured with getdata():
imshow(img1(:, :, :, 1));
imwrite(img1(:, :, :, 1), fname, 'jpg');
Also, for speed reasons, you probably don't want to call imshow() in the loop, but call it once outside the loop to pre-generate the image:
himg = imshow(zeros(480, 640, 3)); % returns the handle to the image
then in the FOR loop, just update the image data using the image handle
set(himg, 'Cdata', img1(:, :, :, 1)); % instead of imshow(img1(:, :, :, 1))
