It really depends on where your at & what the weather is like there. We did this with the MC in Seattle & got mold between the two covers, even with the Dri-Z-air dehumidifiers inside the boat. But here in eastern WA, it's usually so dry, it's hard to find mold growing anywhere. Even when it does rain, it's usually done in a day or two & then dries out again.
If the extra cover makes you happy & feel like your taking care of your baby, so be it. Whatever works. I'd like a carport myself. Or better yet, a boathouse.