Are the two broodies due to hatch at the same time or are the hatches spread out? If so, how much apart are the hatches? Exact dates would help. If the hatches are at the same time, can one hen cover all 12 eggs? If they are bantam eggs, probably, but if they are full sized hen eggs probably not.
Do you have any other chickens, especially adults? Can you cancel Monday’s (today’s) chicks?
What does your set-up look like? How big are your coops and do you have a run associated with them? How big are the runs? “Not very large” doesn’t tell me much. Photos would be helpful.
What is your weather like, now and what’s expected in March? What weather conditions are you going to be dealing with?
I forgot to ask, do yours free range or are they kept in a run?
It’s hard to make suggestions when you don’t know what you are working with. You have yourself in a bit of a bind but there may be solutions.
With those two broody hens you are dealing with living animals. No one can tell you how they will behave. Many people have broody hens hatch eggs in the same nest where they work together to hatch the eggs and raise the chicks. Often different broody hens will hatch their own eggs, imprint on the chicks, and raise them with other broody hens doing exactly the same with their broods. Sometimes it is not a problem at all.
At other times a broody hen may attack another hen’s chicks. That really does not happen often. Sometimes the two broody hens will fight over the chicks, wanting to raise them all and not willing to share. It’s possible a chick could get hurt in this fight, but not likely. I once had two broody hens fight over a nest of eggs. When the eggs start internal pipping they start peeping, talking to the broody hen. When the second broody hen heard that peepping, she abandoned her nest and tried to take over the nest with hatching chicks. They destroyed several eggs in the fight.
What are the chances of problems or success with two broody hens is a very good question, either while hatching or raising them. I wish I had a good answer. It’s going to depend on the personality of the two broody hens, how they are set up, when they are due to hatch, and how much room you have. I can’t tell you what your chances are. Many people are successful with multiple broodies, either in the same nest or in separate nests. Sometimes disaster happens. I can’t give you any guarantees either way.
I don’t know if I can help you or not but I’ll try if you give me enough information to work with.