Actually that's not true. If you insist, they will have to give it to you, 2 embryo transfer. But before they give it to you, they would want to make sure you are fully aware of the risks of having twins, like gestational diabetes, miscarriages risk is higher, pre-term delivery and yes, don't even think of natural delivery, it would be C-sect.
Cos I have 2 boys and before ET, Dr Tan did have a short discussion with me to see if I would be keen to transfer one or two blasto. He did tell me about the downside, but end of the day, he said he would leave the decision to me. I chose to transfer one blasto.
I think it's absurd when they insist to put only one embryo despite against your wishes. There is no regulation to say that you can only put in one.