Warm the butter, olive oil, onions, and curry powder in a large stockpot uncovered over low heat for 15 to 20 minutes, until the onions are tender. Stir occasionally, scraping the bottom of the pot.
Peel the squash, cut in half, and remove the seeds. Cut the squash into chunks. Peel, quarter, and core the apples. Cut into chunks.
Add the squash, apples, salt, pepper, and 2 cups of water to the pot. Bring to a boil, cover, and cook over low heat for 30 to 40 minutes, until the squash and apples are very soft. Process the soup through a food mill fitted with a large blade, or puree it coarsely in the bowl of a food processor fitted with a steel blade.
Pour the soup back into the pot. Add the apple cider or juice and enough water to make the soup the consistency you like; it should be slightly sweet and quite thick. Check the salt and pepper and serve hot.