I have twin boys who attended same primary (same class as only one form intake), same middle school (different class at their request) and now the same senior school (again, different class at their request). Although their needs are different, they are fierecely competitive with one another but at the same time don't want to be compared. I have found that in a large secondary school both their needs are met, they are not compared but treated as individuals. In my opinion, the benefits of going to the same school far outweigh any drawbacks. They are following same curriculum at a similar time so can ask each other questions, they get a similar amount of homework, they have loads of friends - their own from their own classes plus each others, they wear each others school uniform as they wear same trousers, same shirts, jumpers, ties and PE kit, I pick them up from the same place at the same time, when snowing they both share the same journey and I haven't got to worry how to get them home individually, if one is not sure of a procedure at the school, the other one confirms it, if one loses a note from school (frequently) which needs signing and sending back, I photocopy the one that has reached me, sign it and send it back in, both have the same opportunities to go on the same trips etc.
The downsides are few but one is parents evening - trying to get round 15-18 teachers in one night rather than 7 or 8 but the teachers know we have two in the same year and tend to be flexible with us.
They are both happy at their school and although some might find that logistical reasons like getting the boys to school should not be used to justify going to the same school, it is one of many pieces of the jigsaw towards a good family life.
The most difficult thing for us has been waiting for results of two grammar school exams - if one had passed and the other hadn't, or one had passed for the school 11 miles to the south of where we live and the other had passed for the other GS which is 11 miles to the north, what would we have done? Fortunately we didn't have to make any difficult decisions as they both ended up at same school.
My boys are at school with a boy whose twin brother did not get into the school and is at another school. Have no idea how each are doing or what they are thinking but people do manage whether they are at the same school or not. Another set of twins (boy/girl) who are in Upper Sixth now went to different schools because of boy passing for GS and girl not. Both been very happy and have both achieved similar academic success.
Your choice. Just thought I would share my experience with you.