You need the hen to accept the chicks, and switch from sitting on a nest to caring for chicks. You also need the chicks to learn that she is "mommy," so they run to hear when they are cold or scared.
Different people do it different ways.
I have had fairly good results doing it this way:
Set up a brooder with heat lamp, food, water. (Lamp, not brooder plate, for this.)
When the chicks arrive in the morning, put them in the brooder. They can spend the day warming up, eating, and drinking.
That night after dark, put two or three chicks under each hen. Remove any eggs she was sitting on. Leave all other chicks in the brooder. If the chicks come in different colors, pick one of each color for each hen, or a sample of the colors that are available. Some hens object if you add different colored chicks later, so it can help if they see all colors the first day.
The next day, make sure there is food and water near each hen and her chicks. The chicks will probably pop in and out, snuggling with the hen and eating and drinking. The hen will probably stay on the nest this day.
That night, put more chicks under each hen. Either put all the chicks out this night, or you can leave some in the brooder for another day, depending on how many chicks. I might aim for 6-8 chicks per hen this night.
The day after that (second day of chicks-with-hen), the hen is likely to come off the nest to take the chicks to eat and drink. Keep an eye out to be sure this goes well.
The next night (3rd night of chicks), put any remaining chicks under the hens. If all has gone well so far, things will probably continue to go fine from this time on.
If you like to free-range your chickens, I would wait several days after putting in the last chicks, before you let them out to range. The chicks need to learn to stay near the hen, and having them in a pen for several days seems to really help. I don't know what size pen is best, but I know that 3 feet each way works fine, and 4 x 8 feet also works fine. I have not tried a pen bigger than that in the first days.
For the two hens: I have had times when hens successfully brooded chicks in one coop, and times when it did not work. One of the "did not work" was because one hen would call so convincingly that all the chicks tried to get under her, but they could not fit. The other hen was all alone, calling and being ignored. I had to put the hen in separate coops, each with an appropriate amount of chicks, to make sure that all the chicks would stay warm.
Other problems are if one hen tries to drive away another hen, or tries to drive away the chicks that belong to another hen.
If they do well in one pen, I would still provide several feeders and several waterers, to make it easier for them all to eat and drink without conflict.
