Imagine the following situation: someone is cooking a fish. If you neglect the fish, the cat will eat it. If you watch the cat, you can get hurt by preparing the fish. Keep one eye on the fish and the other on the cat. Another variation is: Um olho no gato e outro no queijo (one eye on the cat and the other on the cheese). This is from a mouse perspective. You can use these expressions in any situation where you want to describe that you need to be aware of everything