There is a thread on this board where someone has done that and more or less got it to work (some searching should find it). It will probably work out reasonably well with two identical batteries (i.e. two new ones).
I would recommend against adding a secondary battery with a different usage history as the one already present or replacing an individual battery of the pair; there is control circuitry involved to handle the Lithium Ion battery properly over its lifetime. As you simply tie them together, I don't think that will work out if the batteries don't respond the same at all times; you could wear at least one out quickly and even risk setting it on fire.