One line will be cold the other hot.Thats how A/C works.The evaporator is on the other side of the fire wall and the heater core and box.Sounds like you have a blend door problem.Some use vacuum some cables.get a flashlight and look under the dash as you change modes.I hope its not an auto air syatem.if you dont have a factory manual its pretty much hopeless.even with it its a pain as they had a setup to check the auto air setup.Rocky