You only need to unwrap the low poly model. When you do a render to texture (baking a normal or light map, etc.), it creates a projection cage that surrounds the low poly model. Its kind of like it extrudes each poly that makes up the low poly model. When you render the texture, it grabs all the high details inside the projection cage onto the UV map of the low poly model.
My green thumb grew the tree my Trojan War horse was crafted from. With roses in our pockets we rally round the tombstones. Ashes to ashes, we all fall down.