The lines running through my house have two pairs (four total) wires in them. I believe this is fairly standard. If you remove the jack from the wall and look at the cabling you should be able to tell for sure. If your lines do indeed have two pairs, then the unused pair is most likely what your second line is running on (unless somebody ran all new wires for your second line).
If this is the case, then to add the second line you need to first get a jack that has two connectors on it. Strip the ends of the wires in the cabling, and wrap them clockwise around the screws in the jack (make sure you get the right wires paired with each other). Wrap clockwise because you turn a screw clockwise to tighten it. Wrapping counter-clockwise would cause your wrappings to loosen as you tighten the screw.
The standard for the pairing colors MIGHT be red with green, and yellow with black, though don't trust that blindly, as it is totally from my memory of working on my phone jacks about a year and a half ago.
Alan Johnson