For the honey-citrus simple syrup: Whisk together the sugar, honey, water and lemon juice in a medium saucepan over medium heat and bring to a gentle boil. Reduce the heat to low, add the orange peel and continue to boil without stirring, about 15 minutes. Remove from the heat to cool while you assemble the baklava. The syrup will thicken as it cools.
For the nut filling: Add the pistachios and walnuts to the bowl of a food processor fitted with the blade attachment and pulse until coarsely chopped. Transfer the nuts to a large bowl along with the graham cracker crumbs, cinnamon and salt and mix until combined. Set aside.
To assemble: Brush a 9-by-13-inch pan with some of the melted butter. Unroll the whole package of phyllo and cut it in half to fit the pan or trim to fit flat. (As you work, cover the unused phyllo sheets with wax paper underneath a damp clean kitchen towel.) Lay 1 sheet of phyllo in the bottom of the pan. Brush with butter, covering the entire surface, then lay another sheet of phyllo on top. Repeat this process until you have 10 layers of phyllo, with butter brushed on top. Add half of the nut mixture, spreading it evenly over the buttered surface, then gently press in with your hands. Begin layering the phyllo and melted butter until you have 10 more layers of phyllo, with butter brushed on top. Add the remaining nut mixture, then layer 10 more sheets of phyllo, brushing with butter in between layers. Brush the top with melted butter, then score the phyllo dough into diamonds, cutting about 1/4-inch deep. (Cut 4 strips lengthwise, then diagonal cuts for a diamond shape).
Bake until puffed and golden brown on top, about 1 hour. Ladle the cooled syrup over the top, allowing it to absorb in between pours. Allow the baklava to sit until the syrup is completely absorbed, at least 30 minutes. Separate into bars and serve. The baklava freezes very well.