I don't think they can make you - I think they advise you, but it is your decision. (alto I'm not 100% sure). Sometimes the mainstream school feels it is best for the child to go to special school too. Signing will help a child to speak, it will encourage speech, signing words along side speech. You seem to suggest that by signing yr child will not speak so much - but that is not true.
It is a shame to separate twins, but sometimes that is suggested by teachers even if they are at the same school. I know cases where twins have been separated at 4 years old as they start school and put into separate classes because teachers thought that they would benefit by making them individual and not too dependent on each other.
I think you are anxious about this because they are twins. How would you feel about your little one, going to special school, in a taxi, where he would learn to sign, and communicate with other children that were just like him, if he was your only child. I think you would want the very best for him; and maybe this is the best; go along and have a look; and then decide.
Try not to be anxious around all these decisions, because anxiety can cause a child to be mute. Also, it may be difficult for him at mainstream school, if he is unable to communicate the way the other children can, he may become frustrated and unhappy. Of course, that may not be the case, he may love it at mainstream school, and not worry at all about his speech delay. You know him best, voice your opinion to the speech therapist and to the headteacher at both schools, and then make your decision.