depends on whether your baby has continously sucked into your breast and has swallowed the milk, and if he is with correct latching, and if you are feeding him in correct position. so if he can get enough milk and full for this meal, you can offer another side. however, if he falls into sleep immediately and your breast is still full, when you express your milk, it is still very watery, you should offer the same side so that he can eat the hindmilk.(later milk), otherwise, his weight will not increase very well.