You should make this the night before you plan to serve, so it has time to get cold.
Melt butter and stir in brown sugar. On a parchment paper lined baking sheet, lay Saltines in squares. Pour butter/sugar mixture over the top and bake at 350 degrees for 5 minutes. Remove from oven and pour both kinds of chips over the top. Put back in oven for 3 minutes or until chips have melted. Cool overnight in refrigerator. Break into pieces.