What is a punnett square in which two parents are heterozygous for both traits in a dihybrid cross?

I can show you how to set this up.  

First, you need to figure out what the gametes will be.  Use the letters P (purple), p (white), S (starchy), and s (sweet) to represent your alleles (versions of each gene).  Since both parents are heterozygous (hetero = "different" so they have one of each "version" for each trait), they would be PpSs and your cross will be PpSs x PpSs.  To find the gametes the parents can create, use FOIL - the First letter of each type, the Outside, the Inside, and the Last.  So these would be your gametes:
F = PS
O = Ps
I = pS
L = ps
Since both parents are the same, both produce the same four gametes.

Now, set up your square.  Because each parent can make four different gametes, the square has to be 4 blocks x 4 blocks:

.....|PS|Ps|pS|ps|
PS|
Ps|
pS|
ps|

Next, combine the letters from the top and side into each box.  Keep like letters together, and put any uppercase letters first, so you can keep track of dominant/recessive phenotypes if you need to know this later.

Last, figure out and count any genotypes and phenotypes for any questions you might have about expected ratios.