Preheat oven to 400 degrees. Prick both spaghetti squash all over with a fork and microwave each one for about a minute, to soften.
Slice squash in half lengthwise and scrape out seeds. Place squash halves cut side down on a greased baking sheet. Bake in preheated oven for about 35-40 minutes.
While squash bakes, prepare the rest of the filling. In a large pan, cook the sausage over medium heat until browned and crumbled. Remove from the pan and place on paper-towel lined plate.
Using the grease left behind by the sausage (or supplementing with olive oil), saute garlic and kale until kale begins to wild and garlic begins to brown. Season with salt and pepper, to taste. Add cannellini beans, sun-dried tomatoes, and parmesan and cook an additional minute or two. Place mixture in a large mixing bowl, along with cooked sausage.
When squash has finished baking, remove from oven and scrape most of their contents into the mixing bowl, leaving a small border of flesh. (Be careful, squash will be hot!) Mix squash strings, sausage, and kale mixture thoroughly.
Divide the mixture among the four squash halves and sprinkle with mozzarella. Place back on the baking sheet and return to the oven for an additional 10 minutes, or until cheese has melted and filling is warmed through. One serving equals half of a squash boat.