Another non-twin parent here and, possibly even worse, I'm a teacher who has taught twins and triplets both separately and together
What matters is not the school's policy or the most recent research but that a decision is reached that will enable your DC to thrive, learn and have a happy time at school. If that means putting them in a class together for Reception then so be it but it would be wise to find out from the school if they mix cohorts at the end of Reception or at any other stage so, if you later feel that your children would benefit from being in separate classes there is an opportunity for that to happen without a single child being moved from one class to the other.
The schools I've taught at always consult the parents and children themselves and try to accommodate those parents' wishes for those particular children. As a result I've taught twins who were in the same class throughout primary, some who started off together but whose parents asked for them to be separated on joining Y1, some who've always been in separate classes and triplets who've been split with 2 in one class and 1 in the other (parents opted for this and chose who went where).
If the Reception teachers run their classes very differently, take the children on different trips, cover topics at different stages of the year (to share resources) then having your twins in separate classes may be a bit of a headache for you on an admin/organisational level but many parallel classes will plan for similar opportunities to happen at the same time in the year so this may not apply at 'your' school.
I think you need to ask the school how they would ensure that both children received the same opportunities if they were in different classes, talk about the possibility of having them together for Reception but possibly separate them at a later stage if it would be beneficial and, above all, let your children see the school in action and ask them what the ideal set-up would look like to them.
Another option is to go on the waiting list for a single-form-entry school.... friends of mine with twins have chosen smaller schools precisely so the issue of being separated didn't arise.