I have only been breeding for 30 years, so this may not mean much.
I have weaned foals together and I have weaned single foals alone.
I have never had any problems with either method.
I wean based on different things. Independence of the foal, if they are bringing their mothers down.
My oldest foal is a month old, he will be weaned about 90 days, his mother is older and gives 200% to her
babies, she will begin to lose weight soon, no matter how much she gets fed. She has always been this way and I
have always weaned her foals at 3 months. She also begins to wean them about 2 1/2 months on her own.
Knowing the mare and foal is key. Last years foal was a single foal, I chose a step down method, starting separating her at night from her mother, then after a week or so, sent her mother off. She fussed some, until I moved her back to the stall she had been born in and she was fine.