Hallertau is only 5-6% AA,so it has a low bittering value. Typically an aroma hop. Magnum is a German hop with higher bittering value of 12-17%AA. It is prized for this,& it's aromas of spice & citrus.
So maybe use the Magnum for bittering,The Hallertau for flavor/aroma?...The two have flavors that are similar,so they should work well together.