It's totally dependent on the twins in my experience (Reception teacher).
I've taught twins in a small village school (identical girls) who were absolutely fine being in the same class all throughout their primary school days. The small size of the school meant that siblings in consecutive year groups were also together every other year. Some siblings were fine with this. Other children probably would have been better off in a bigger school, where they could have been in a different class to their sibling.
I also taught boy/girl twins in the same small village school. They probably would have been better off apart, in hindsight, as she was often annoyed by her brother's presence and could have done with some space from time to time (and he probably would have benefitted from not having his every misdemeanor tattle-taled to mum by his sister at home time!). Mum didn't drive though, so sending them to a different school wasn't an option.
I've since worked in larger schools, where twins have always been separated into different classes. Personally I think it's good for children to have some space away from their twin, to develop their own friendships and grow as an individual. However, I know some parents are initially disappointed by this and ask for them to be kept together. Our reasons are always thoroughly explained and I think most parents would agree afterwards that it was probably the best thing for their twins.
At the end of the day, they're highly unlikely to work in the same work place as an adult, or live together forever. A twin bond is a very unique and special thing, but it's also important to allow them to be individuals too. Otherwise they'll spend their life being 'the twins' who are always lumped together, rather than seen as individuals in the eyes of their peers.
My advice would be to look around all your options. If you have the option to go to a 2 form entry, where they will be in separate classes, it could be a great opportunity for them. If you think a one form entry is the best school out of your options though, I'm sure it won't be a problem them growing up in the same class at primary school.