To be honest, if you’ve got 2 dogs who initially like food then start turning their noses up at it, the most likely thing is you’re giving them too much of it. Ok, if they refuse to eat it for days at a time or start losing weight, its time to change diet. For the most part though, if a dog’s hungry, it’ll eat what’s put in front of it, especially if there’s more than one in the house.
It always fascinates me that people think a food must be good for a dog because they wolf it down quickly. The reason a lot of tinned foods are so palatable is due to salt/sugar content – the equivalent of fast food for dogs. As for protein levels, dogs are Omnivores, not carnivores and high protein levels can lead to more problems than they solve. Yes, in rare circumastances, you can get intolerances to different ingredients including grains but, for the most part, they are fine.
I could go on (and on, and on :) ) but my advice would be pick a quality food (Royal canin would be my choice but most of the above suggestions are ok and Chappie is alright if you’re on a bit of a budget) and stick with it. The more you chop & change, the more fussy you make your dog – it’s quite easy to train a dog to be fussy by “rewarding” them for not eating what you first put down.