What an interesting quesition and what intersting answers you have gotten.
My experience in this instance is as a preschool teacher for 8 yrs. actually had a class with 2 sets of triplets in it one year. and also as a mom with a summer bday boy and a winter girl 18 months apart.
The key to my answer is going to be your DD's personality. I think there is no question in my mind that YES you should wait a year to send DS to kindergarten, and I am of the philosophy that having a child evaluated for any sort of special needs is a win win, either you find out what the problem is and can start to fix it or there is no problem. so win win.
I obviously haven't observed your son so i don't know if maturity is the only issue or if there might be a bit more that might be something to check into. If he can pay attention to a book, or puzzle or building with blocks when he Wants too then he is probably fine. but you would know best if there might be a bit more to it and either the school or your dr can help you out.
SO i definately say wait with him, As for DD unless she is uber confident and uber genius, I don't see it hurting her to wait a year as well. If she isnt' overly percocious i think she would be fine and you really don't need to worry about her being "bored". The teachers would give her reading material that fits her level. and if she is that mature she would make a great role model for the younger kids in the class. and probably have a blast. not to mention life would be much easier for you logistically. I personally feel separating them into different classes would probably be a good idea so they aren't competing for friends, honors etc, they would still see each other at recess and lunch etc. The one poster whose husband was a twin and comented differently was interesting i usually don't hear that perspective.
So my vote is to hold them both back as long as it is reasonable considering birthdays etc. I see that scenerio less emotionally harmful for both than advancing one at this stage.
If it helps, when i was deciding to hold back my late july Smarty boy. several people told me how yes he would be advanced for while but by 6th grade when it was time to learn all new stuff having the year of emotional maturity would be better for him. than worring about him being bored in kinder. It's a new playing field in middle school and I htinkw e made the right choice, he is in 3rd right now and being bored has never been an issue he gets to go to a gifted type program once a week he gets to read at his level and it's all good.
Good luck with what ever you decide. I hope you have some twin resources like message boards or community groups that can help you decide.