Hello,
I am sorry to hear of you plight.
And very sorry I am not a good experienced source to help you.
I empathise only in this situation as I too had some urgent need earlier in the year, and had little help for what seemed like days when I had a sick chicken, who I ended up saying goodbye to.
Keeping each hen safe is important, and you are doing this - safe and secure, fed and watered etc.
But it is the freedom that you are seemingly dealing with. Keeping the 'broody' one safe, and the 'opportunistic' bully away.
Unsure of your weather, shelter, coop situation. And what changes if any that have brought about this change in caring friendship this pair have had with each other.
I could lean toward, being winter, and lesser food to forage for, and one getting tough on the other because it is hungry and fending tough for itself and to meet it's hunger! Normal for them - survival of the fittest.
Unsure of anything more.
Do hope that someone with great expertise in years of experience will come across your plee for help.
thinking of you and your situation.