I had b/g twins in the same class in Infant school.
My dd was a typical girl, paid attention, always tried to please etc, the teachers in every year loved her, the teacher used to compare my kids in every aspect of their schooling - so while I tried my best not to compare my kids the teachers happily waxed on and on, comparing them at length like they were some kind of scientific experiment - it wasn't nice and it's not healthy - they are individuals - it shouldn't need to be pointed out.
I couldn't go in and have a conversation with the teacher about my ds without them bringing up my dd - sometimes to reflect on how easy she was - sometimes to bring her weak points into the meeting for discussion too, it was very frustrating. They always related the success of one to the failure of the other - the comparisons annoyed me beyond belief - I wanted them to be treated as individuals - with individual attention to their progress at times the teacher mixed up them up in her descriptions. She even asked my dd to tell tales on my ds - which caused no end of problems at home.
One woman with twins at the school said to me that I was lucky because she had already trained the teachers in dealing with twins in the same class - she said she went through a hellish year - daft things like sending home the class mascot for both of them to share - her girls didn't understand why they had to share the teddy and no one else did.
My kids are very close despite being b/g and enjoy each other's company. They were separated in nursery and played separately but when in school in the same class they became very close again. Not helped by the rigid ability setting in the class - they were judged to be the same and therefore had to sit at the same table for most of the day.
My ds liked to play with the girls in the class who he sat with but they were never seen as his friends, always as dd's and he found that hurtful, confusing and very isolating.
We moved school so they could be in separate classes. Before they never discussed their days with me, now they talk at length with me and with each other about what they get up to - it's really lovely to hear them get excited about their day. They still find each other at playtimes - they play with a big group of boys & girls kids from both classes, they have their own friends and they have shared friends. Parents Evening is a joy as I speak to a teacher about each child and they make no comments about the other one - I can't tell you how nice that is. And it also means that I'm not seeing the teacher twice as much as everyone else as I'm only seeing her about one child.
Changing schools over it was scary but imo it wasn't a great school anyway - I know it can work for some, teachers can help, some are more sensitive to the difficulties and don't spend their time with you comparing your children and some kids behave as if their twin doesn't exist - some get incredibly competitive - we didn't have that but I know it's a common problem.
My advice don't do it - or at the very least don't choose a school a single entry school.