using Arc Map
Go to Layer
Properties>Symbology
Choose -> Multiple
Attributes
Choose -> Bivariate
renderer
Select the two attributes you
would like to map
It is a bit buggy
Probably a good idea to
stick to 4X4 grid
Pick Two colors that will
combine well
Click apply
Export Legend
Go to Layout view
Insert Legend
Make sure your Layer is in
the box on the right side
Resize legend and export