Bring water and salt to a boil in a large stock pot. Cook pasta until al dente (still slightly firm). Reserve 1/2 C pasta water, and drain. Set aside.
For the Shrimp:
Meanwhile, in a 12-inch skillet, melt butter and oil. Toss raw shrimp with cajun seasoning and salt. Once butter has melted and is hot in the pan, add the shrimp and sauté until well browned on both sides and cooked through, about 2 minutes per side. Remove shrimp from sauté pan and set aside in a separate bowl.
For the Pasta:
Melt additional 1 Tbsp butter in the same sauce pan as the shrimp were cooked in. Add onions and jalapeños and sauté, stirring often, until starting to soften and brown, about 5 minutes. Add the garlic and sauté until fragrant, about 1 minute.
Add the cream, the cajun seasoning, 1 tsp salt and 1/2 tsp pepper to the pan and bring to a hard simmer. Cook until the cream reduces by a quarter and thickens, about 2 minutes. Add the diced tomatoes, reserved pasta water, cooked shrimp and cooked linguine, and toss over low heat, to incorporate the sauce and finish cooking pasta, about 3-4 minutes.
Remove from heat and stir in the pepper jack and parmesan cheeses, as well as the basil and parsley (if using). Toss to blend until the cheese is fully melted and sauce is smooth. Serve immediately.