Due to the fact that all dogs are different, in terms of what they like and what works for them (same with humans), it’s going to take a bit of trail and error to find what works for you. Thankfully, most good foods will offer free samples, which you can bring to your dogs to first make sure they’ll eat it. After that you can purchase small bags to feed them, and monitor how they do with it (firm stools, good weight, etc).
I’d go with a food that is grain free and higher in protein, whether that be kibble or whatever type. A dog’s system is designed to process protein, rather than carbohydrates, so your big dog won’t gain too much weight due to grains and carbs, and your little dog will be able to get more out of the protein-rich food and put on some weight.
What it will come down to, I believe, is that you can’t just free feed them. Meaning that you simply keep their food bowls full, and let them eat at their own leisure. If you think of them as children, you’ll know that they aren’t going to regulate how much they should or shouldn’t eat… that’s mom’s job. I know it can makes things tougher, but as long as you’re having weight issues then I don’t see any way around it.
