As Nitrogen said, isolators are typically used for running things from a second battery when the engine is off. You can run that battery down and still start your engine with the other battery. When the engine is running both batteries are connected together and the plow would be pulling current from both anyway. So I see no need for an isolator or relay. The lights are dimming because the plow is pulling a huge current load from the battery. With two batteries in parallel, you have twice the capacity to pull current from so the battery voltage shouldn't dip as much and you'll see less light dimming. It may not go completely away but should be improved. Make sure the plow is connected as close to the battery post as possible - preferably at the post.