Mine are boy/girl and started last Sept. and I split them. The school were great and gave us the choice. I think I asked on here for some feedback at the time.
When they were younger I was always sure I'd keep them in the same class. Things that changed my mind were:
Although very close, they played apart quite happily making their own friends, so personality wise I knew they'd be ok with it.
I thought if they ever had to split them later ie in junior school or older, it would be less of a trauma if they were used to it anyway.
I wanted to give them their own space at school and not feel in competition academically in the same class.
I didn't want them to be summed up as 'the twins' all the time (not such an issue with B/G it's true) Some mums - not knowing they are twins, have asked just one to a birthday party. A couple have apologised and said they'd have asked them both if they'd known. But this is just fine and partly why I split them; so they'd learn that they don't do everything together all the time.
Give them a different 'day' each iyswim, and new things to chat about to each other after school. Also to have 'their own' teacher. Twins, well mine, seem to like to have their 'own'.
They seem very happy and settled with it. My ds especially as he has a very close friend in his class from pre-school anyway.
My dd is still summing it all up I think and hasn't made a very close friend yet - just more general ones. The little girl she used to like best at pre-school went to another infant school. The school is small though and only has two reception classes which mix together often anyway.
I think it depends on personalitites though. Mine are quite easy going and confident.
Downsides are mainly on me...
Two sets of teachers to deal with. Not a big problem - they're lovely, but it's twice the correspondence etc.
Now the biggie...The birthday party! OMG! I ended up negotiating the entire reception year because with one in each class you have twice the children and mums to consider.
It is a big choice and I worried about it a lot. They have taken it all in their stride - so far anyway.