Cook the pasta according to the package directions. Once cooked, reserve 1 cup of pasta water and drain.
Pat the shrimp dry with 2 paper towels, then transfer them to a medium bowl. Add 1 tablespoon of olive oil and 1 ½ teaspoons (½ tablespoon) of the Cajun seasoning. Toss the shrimp in the olive oil and seasonings.
Cook the shrimp:
Heat a large skillet over medium-high heat. Add the shrimp in a single layer without touching and cook them for 1 to 2 minutes per side. Avoid overcrowding; cook in two batches if needed—otherwise, the pan will steam and the shrimp will release watery juices. Shrimp are done in 3 to 4 minutes total, once they’re no longer translucent pink. Remove them from the pan onto a plate and repeat the batch if needed.
Add the sliced sausage to the empty pan and cook it just until it's heated through, about 2 minutes. Remove it from the pan onto the plate with the shrimp.
Make the Sauce:
Add the remaining 1 tablespoon of olive oil and the butter to the pan, along with the seasoned tidbits. Use a spatula to melt the butter, then add the onions and sauté until soft and translucent, about 3 minutes. Add the garlic, and the remaining ½ tablespoon of Cajun seasoning. Stir, and continue cooking for 2 more minutes.
Add the half-and-half and Parmesan cheese and mix until combined. Bring the mixture to a simmer and then reduce the heat to medium-low and simmer the sauce for 5 minutes.
Combine:
Add the pasta, cooked shrimp, and sausage to the skillet, along with ½ cup of the reserved pasta water, and toss to combine with the creamy sauce. Slowly add the remaining pasta water to make it more saucy.
Serve this shrimp and sausage pasta topped with Parmesan cheese.