A second tank will be good if you are going to be doing any sanding or painting (that is where you need a big tank).
Those are not the greatest compressors in the world but for taht price and that warranty you cant go wrong, just make sure you keep oil in it.
use a fitting and 3/8" or larger hose to connect the compressor to the tank (use couplers on each end so you can use the otehr 8 gallon tank as an "air pig" if you so need). Then hook a regulator on the new tank.
Personally i wouldn't bother doing this, i'd just buy a bigger air compressor, save the 8 gallon tank for a nice OBA system...

Oh, 8-13 gallon isnt big, 120 gallon is big <img src="/forums/images/graemlins/scared.gif" alt="" />