Do I feed my two dogs the same food??

Question
I have a 3 year old 100 pound German Shepard, and a one year old 40 pound beagle/lab mix.
I was feeding the shepard large breed formula (purina proPlan), and the beagle/lab puppy chicken and rice formula(also purina ProPlan).  I was sick of the dogs getting into each others food all the time when I wasn't looking so I decided when the puppy turned a year old I would ween them onto the same food (to avoid the constant diarhea!)
I decided on Lamb and rice formula, but it seems that neither dog likes it, and will barely touch it, and don't know whether to stick it out or try a new brand.  I really wanted to continue the shepard on the large breed formula because he is massive, but didn't think it would be adequate for the beagel/lab.
Is there a kind of food that would meet both of their needs, or is it pointless in trying to get them to eat the same thing and should go on feeding them their own.

Answer
Hi Sarah,

What I would recommend for these two dogs is to switch both to a premium food. You can use the same food, just feed less to the smaller dog. I like FrommFourStar, Innova, some of the Natural Balance, and Orjen or Wellness CORE. IS it an option for you to use a premium food? Chances are they will greatly appreciate the change and you won't need to monitor as you have been doing.
Cat :)