Bring a large pot of salted water to boil over medium-high heat. Add the shell pasta, and then return to a boil. Cook, stirring occasionally, for 9 minutes. Add the tortellini. Cook, stirring occasionally, until the shells and tortellini are al dente, about 3 minutes. Drain; set aside.
While the pasta is cooking, melt 4 tablespoons of the butter in a broiler-safe Dutch oven over medium high heat. Cook, stirring occasionally, until the butter smells nutty, about 2 minutes. Whisk in the flour, and cook until toasted and light brown, about 2 minutes. Reduce the heat to medium; gradually whisk in the milk. Cook over medium, stirring occasionally, until the mixture can coat the back of a wooden spoon, 5 minutes. Add the cream cheese, mustard, salt and pepper. Cook, whisking constantly, until smooth- 1 minute. Gradually add the two cheeses, whisking in between additions, until smooth. Remove the pan from the heat. Stir the drained pasta into the sauce in the Dutch oven. Toss to coat. Set aside.
Preheat the oven to broil with the rack about 8 inches from the heat source. Microwave the remaining 1 tablespoon of butter in a small bowl (until melted- about 30 seconds). Add the panko to the melted butter; toss to coat. Sprinkle the buttered panko cumbs evenly over the pasta mixture. Broil until brown and crispy, about 3 minutes- watching closely to make sure it does not burn! Sprinkle with chives before serving.