What is wrong with my 2004 Mitsubishi Endeavor windshield wiper?

It appears to be two places for a windshield wiper arm directly in the middle of the windshield side by side. The knob or motor on the right spins when the wipers are on, but the one located closest to the penger side which has a windshield wiper arm attached to it does not move at all. What is the purpose for two in the middle, do I need a new motor? Can I just take the arm off and attach it to the knob that is working or will it collide with the driver side wiper? lastly where can I get a replacement arm?