Don't know about how to use it, but I have an idea why it's there: I read that the Durango is built on or patterned after the Mercedes GL platform. So, since Mercedes is into using so many diesel engines in cars and such, makes sense there would be provision for two batteries, one under the passenger seat (already there) and one under the driver.
I looked at using it on mine, but so far have declined due to difficult access.