I had the same question when I got Archie last fall. Since my boy free feed, that was even a bigger problem. The only thing I was able to do was mix the regular and puppy. When Archie was really small it wasn't too much of a choice for him, the puppy was smaller so he would take that over the larger kibble. But Andy and Barney ate the puppy too
It hasn't really hurt them and soon I will be switching to all adult soon. Other than seperating them there really isn't too much you could do...