We have 3 cats, and one day they tried tinned dog food and are now absolutely addicted to it.
We occasionally feed it to them as a treat sometimes and they love it. They've been very healthy for the past few months so it doesn't seem to be having any negative effects, and their main meal is cat food as always.
I think the important thing is to make sure they eat mainly cat food to get all the necessary nutrients. And make sure the dog food is good quality, also important for your dogs health. If in doubt you can always ask a vet.