In a small saucepan, over medium heat melt 1 stick of butter. Add the garlic. Reduce the heat to a very subtle simmer. Simmer, until the garlic is very fragrant, about 5 minutes. Remove from heat and stir in the salt, tarragon and parsley. Remove from heat, cover and set aside while you prepare the fish.
For the Lobster Tails and Shrimp:
Preheat the grill to medium (about 400-450 degrees Fahrenheit). (*see notes)
To prepare the lobster, take a good pair of kitchen shears and cut down the center of the spine (or the top of the tail) lengthwise.
Place the lobster tails and the shrimp in a large bowl. Drizzle with avocado oil and season with salt, pepper and old bay seasoning. Gently toss until coated evenly.
Place the cedar planks on the grill to get hot, about 3 minutes. Once hot, place a lobster tail on each plank. Cook, covered, for 5 minutes.
After 5 minutes, push the tails to the edge of the plank then evenly divide the shrimp amongst both planks. Cover and cook for an additional 6 minutes, flipping shrimp halfway through, about 3 minutes, until they are no longer opaque.
When the lobster and shrimp are cooked through, carefully remove from the grill (I use a good pair of tongs) and serve with the garlic-herb butter. Enjoy!
*for best results, I prefer this on a charcoal grill; however, any grill works!