Combine sugar, water, port (or wine), cinnamon and nutmeg in a medium nonreactive saucepan (see Note); bring to a boil. Add cherries and cook for 1 minute. Stir in cranberries; return to a boil. Reduce heat and simmer until about half the cranberries pop, 10 to 12 minutes. Remove from the heat.
Stir in walnuts and orange zest. Let cool completely. (The marmalade will thicken as it cools.) Serve at room temperature or chilled.