One thing that's become apparent to me, is that while I"m sure that any dog food that meets the AAFCO standards would have sufficient nutrients (and so, barring medical issues, you could feed any adult dog food to any other), there is another issue, apart from nutrients. I've noticed that the protein and fat content of the small breed dog food is significantly higher than the large breed (29 compared to 21 percent protein, and 17 to 12 percent fat--Pro Plan Focus Adult Small Breed vs. Hills Science Diet Large Breed). And so while both brands should have the nutrition that any adult dog would require, it would be much easier for my Saint Bernard to get fat by eating the small breed dog food.

And so I will continue to fight to get those rascals to eat as little of each other's food as possible, knowing that small amounts here and there probably won't matter.

 

For puppies it is very important that large breed puppies are fed a LB puppy food due to strict calcium requirements. For adults it's not a huge deal. Yes small breed food has more fat so your St. Bernard could beef up quick eating it. But a large breed food most likely will leave your small dog malnutritioned. A normal adult dog food would be a happy medium. Something with a moderate amount of fat and protein.
And while I know it's not what you were asking, I feel it's worth noting that Science diet and pro plan are both terrible foods full of by-products and fillers and preservatives.
Both dogs would greatly benefit from a higher quality holistic food with no by-products and fewer grains.