I always brood in my garage. You're right that as they get bigger they get messier, and then they also SMELL. I love ducks and I love ducklings, but man I hate brooding them lol. With two, you might not have as much of a mess on your hands. I have 19 right now and I guarantee I will have at least one more round before I'm done lol. My garage has a back room that is insulated. So even when it's extra cold outside, the temp holds steady. When it's extra cold, I either add a second heat lamp or I lower the lamp I have on. This way I know the peepers can get warm no matter how cold it is. Could you get a large cardboard box, or larger totes to keep your brooder in? This would just add another barrier between the babies and the cold.