Dog got diarrhea, more details in question

Well, the pumpkin and yogurt could have upset their stomachs, especially if they are allergic. And likely the sudden change in food is wreaking havoc on his system. But I would think that one of the dogs had an infection and ped it on to the other. Maybe a flu or something. I doubt it's anything serious, especially if caught in time, so it's a good thing you are taking them to the vet. Keep them hydrated.