* Nutritional needs: Mice and guinea pigs have different dietary needs. Mice need more protein and less fiber than guinea pigs. Feeding a guinea pig diet to a mouse could lead to health issues, and vice versa.
* Potential for illness: Sharing food could lead to the spread of bacteria or parasites between the animals, especially if the food isn't fresh.
* Dominance issues: The guinea pig might bully the mouse away from the food, leading to the mouse not getting enough to eat.
The best thing to do is to feed each animal its own, appropriate diet. This will ensure that they get all the nutrients they need and stay healthy.
If you're concerned about your animals' health, please consult a veterinarian.