Well, I certainly wouldn't consider myself an expert by any stretch, but if they had been doing well on the same food up until recently then I may consider something other than the food being the issue. It would be odd for two dogs to develop an ingredient sensitivity at the same time, though I suppose it could be possible. Has anything else changed since they started having problems? Less exercise, some new stress, eating something outdoors that perhaps has upset their stomachs?
Maybe try adding a spoonful of pumpkin to their meals and see if that helps. You might try cutting portion size back a bit too. Over feeding can cause loose stools as well. As long as it isn't full diarrhea I wouldn't worry too much. If it doesn't pass by the time you're getting close to the end of the current bag then maybe look at switching foods again.