Both of them should be on a puppy food until 12-18 months with the size breed they both are and their high energy levels ( if you want an All life stages food - keep reading) Typically you can wean off puppy based food and transition to adult around 9 months of age but since you want them to both be on the same food then keeping them on a puppy formula until the heeler mix is at least 9 months will not hurt either.
Puppy food is a high fat, high protein diet so if you notice that your shepherd’s beginning to gain too much weight then switching to a food Formula that is acceptable for “All life stages” Such as ‘Canidae’ Might be the best and most affordable food for them to both enjoy.
Alternatively, if you would like to keep them on puppy formula that is relatively inexpensive and Still has all key benefits - as a veterinarian technician myself, I’ve always recommend Purina Pro Plan Puppy. Most of Purina brands are simply put “garbage”. But the Pro Plan formula is one of the best for your buck.
I would definitely NOT use kirklands Costco brand as it’s ingredients are mainly fillers and have been proven to cause many health issues when used for an extended period of time.
Also make sure once both your dogs are weaned off Puppy formula and if you choose not to try Purina Pro Plan, Try to stay away from high protein content foods. Many foods that claim to be “grain free” are high in protein and not to mention expensive. The cost is not the issue in this case it’s the high protein levels. High protein diets ingested daily in dogs over 12-18 months of age are prone to UTIs, kidney stones and other urinary issues as they cannot properly flush the proteins from their bodies. This is including any RAW food diets - not to mention they’re expensive, give dogs numerous amounts of oral health problems and are often recalled due to their risk of salmonella poisoning to humans. So expensive isn’t always better and grains are meant to be incorporated into their diets because dogs are NOT carnivores. They’re omnivorous like us and need grains, nuts, berries etc. In actuality only about %8 of dogs actually suffer from grain allergies.
If you are still not convinced on the type of food you should feed please just read the back ingredients of the dog food of your choice and read off the 4 first ingredients as that will be the foods primary source contents. If they’re unknown chemicals or weird synthetics - try a different bag. And that is the rule of thumb for any dog food you choose.
I could go on forever but I will leave you with this and wish you the best of luck!!