In this case you would use a 16 box punnet square not a 4.
The punnet would look something like this
http://www.ralphdavisreptiles.com/ma...owxdh_snow.asp
except for being double het for snow you could look at it as het for leucy and het for super cinny. I hope that makes sense. I couldn't find any punnets with two co-dom traits.