In a skillet melt 2 tablespoons of the butter over moderately low heat, add the cream, the Cognac, and salt and pepper to taste, and bring the mixture to a boil.
Add the morels, simmer them, covered, for 10 minutes, and keep the mixture warm.
In a kettle of boiling salted water cook the porcini fettuccine and the scallion fettuccine for 3 minutes, or until it is al dente.
While the pasta is cooking, in a large deep skillet melt the remaining 6 tablespoons butter over low heat.
Drain the pasta, add it to the large skillet, and toss it with the butter, lifting the strands.
Add the morel mixture, 1/2 cup of the Parmesan, and pepper to taste and toss the mixture well.
Alternatively, the 2 kinds of fettuccine may be cooked and sauced separately.
Serve the pasta on heated plates with the additional Parmesan.