It's not that the parent is requesting "special treatment" to allow the twins to remain together. Even if she does want them together she's wanting to be consulted. Any school worth their salt would give a parent that respect and present the value in separating them. And yes, just the very nature of them being twins makes it a decision whether to keep them together or separate that you don't have with other situations.
J., it sounds like you have had a bad experience with this. It IS frustrating when the schools make those decisions without consulting the parent as if they are the ones in complete control. Unfortunately, that is the attitude of some school districts. It's ridiculous to have to get a law passed to force a school to treat parents with the respect and value they deserve.
I am a twin who has experienced this. I grew up in a family that moved a lot due to my dad being a construction worker. I remember vividly the first time my sister and I were separated. It was unexpected and very traumatic being in a new school, new state, new living environment and not having that support we had always had. It's one thing to be able to prepare for it. It affected me in ways I didn't realize until I was an adult.
Having said all of that I will also say I have been a teacher in a public school and I understand that there are some parents that are extremely difficult to work with either because the parent is over the top protective or just doesn't care. I've seen some stay at home moms that won't get up to fix their children breakfast nor will they them to school early enough for them to eat breakfast at the school and it's free.
Also, there are some multiples where one of the children does the work for the other(s). Therefore it is in their best interest to separate them. However, that doesn't give the school the right to do it after you leave. You are the mom. It's easy enough to tell which parents are involved and which don't care.
You may just need to be on your toes and be thinking ahead of them in the future. Direct, respectful communication on everyone's part goes a long way in heading off problems. The bottom line is everyone should want what is best for the children. If the separation was especially traumatic for your twins I hope you have helped them work through it. If so, they are probably stronger for it. I know from experience it is easy for one to be the dominant personality and the other enmesh with it. Once I revisited the experience with adult understanding I realized it was part of the process of me becoming an individual apart from my twin. Which was a good thing.