Using two bags at once with dog

I did the same thing with my 27 pound dog at 16* and it worked great.
I used my 15* bag and a homemade 15 ounce quilt with 9 ounces of down.
I also had on a very warm jacket, but I never got even a bit cold.

Bag weight was 2 lbs 2 ounces, so 3 lbs 1 ounce total.

I made an 8 ounce sleeping bag with pad after this for my dog, only because I can now just throw a layer of my bag over her instead of her having to be completely engulfed on my body.

It depends on what works for you. Inviting a wet dog into my bag is not something I would contemplate, so I throw a custom 'quilt' over the dogs (I carry a separate mat as well). They are sensible enough to stay under the quilt if it's cold, but not all dogs get the hang of this. If you want to sleep with your dog, then a quilt is the way to go…

Absolutely.

I won't comment on the dog inside in winter, but the idea of an over-bag, or over-quilt is not uncommon. I am actually going to be doing this as a side-project this winter, using a 0 F bag along with a over-quilt to take it into the minus teens.

just make sure the quilt has a large enough footbox to fit over your bag, and that it is long enough to make it up to your chin at least. You may need to bolster your head warmth with a fleece beanie, maybe even a down-filled bala depending on how cold you expect.