We split our ID twin girls in P1 (reception) and while I don't regret it, it did show us that they were better off together. We put them back together for P2 and it was the best thing for them. They are now P3 and still better off together. Teachers agree.
I will say that our girls have had a tumultuous few years with overseas moves and serious illness within the family, so their situation is perhaps not the norm, but regardless of that, they do need each other. I do wonder if we hadn't moved back to the UK, and had stayed in their small private school in the US, whether they'd have been better off separated. (Which was our plan for kindergarten had we stayed) I think they'd probably have been happier separated there than they were here, but that was a very different reality than here.
P1 didn't wreck them in any way - they coped and like I said, we don't regret splitting them for that year, but we did learn a lot from it - as did they. We requested putting them back together again (after they had been asking all year) and the school agreed. When it was time to move up to P3 their teacher came to me and asked my preference and told me hers was to keep them together.
My DH has the potential to move to London for work, and being in a school with a policy of splitting twins is one of the big reasons we are very hesitant about a possible move. While P1 didn't damage them I don't think, I do think splitting them now - especially coupled with a move - would potentially do a grata deal of damage and I'm not prepared to take that risk.
I know my DTs though and I know what's right for them. You know your DTs best.