So pleased you did that. Go, Joy!!
You´ll probably remember all this: I´ve had a goose that hatched out 3 gozzies, the gander tried to look after them, but the goose refused to leave the nest. I lost 2 of those babies, as she let them get cold at night, she just wanted to brood eggs! Stupid goose. So, I took some eggs from her sister and put them under the first goose and she sat on them till hatch, and this time I made sure there was nothing left in her nest, not even the tiniest stone for her to brood, and she did fine. So, she actually sat 2 months. I just made sure she had plenty of food and water. Another time was a muscovy, just like yours, became broody, but I wanted to keep her broody until she could sit on some goose eggs. I took her eggs away and I put fake eggs under her to keep her in the mood, and then after a few weeks when the goose had laid some eggs, I popped the eggs under the muscovy and she sat there the further 4 weeks to hatch them. I left the goose to get on with her own brood of eggs, then took the tiny gozzies from the muscovy as soon as they hatched, leaving her with acouple of dud eggs to keep her quiet, and introduced the gozzies to the goose momma when she´d hatched hers out a little later. all a big happy family, and after a bit I took the dud eggs away from the scovy and she started laying again after a bit.
So, after all that waffling, you could always leave the 2 eggs with the scovy until hatch, I´m quite sure she´ll do a good job, and maybe Missy will sit on the other two, maybe not, but you can introduce the duck-hatched gozzies to Missy later for her to adopt. I´d be inclined to put all 4 under the muscovy, then if Missy goes seriously broody, just let her have 2 of them back again to continue incubating. That way you´ll get all the gozzies the same age. Just my thoughts on it all,