Fill a large pot with lightly salted water and bring to a rolling boil over medium heat. Stir in fettuccine, return to a boil, and cook until tender yet firm to the bite, about 8 minutes. Drain and keep warm.

While the fettuccine is cooking, heat 1 cup butter and cream in a large saucepan over medium heat. When butter melts, add Romano, Parmesan, 1/3 of the garlic, salt, and pepper; cook and stir until cheese melts and sauce thickens, about 5 minutes. Remove from the heat.

At the same time, heat remaining 1/4 cup butter and olive oil in another saucepan over medium heat. When butter melts, add onion; cook and stir until softened and translucent, 2 to 3 minutes. Stir in mushrooms and remaining garlic; increase the heat to medium-high and cook until mushrooms are soft, about 2 minutes. Add shrimp and cook until bright pink on the outside and opaque in the center, 1 to 2 minutes.

Fold shrimp mixture into sauce. Add drained fettuccine and toss to coat.