if tie on highchair for whole day, DEFINITELY will affect your bb's ability to turn (he or she can't move, not to mention turn!!). i think you should place bb on bed / playmat / planpen asap...
It is not healthy for your baby to sit on highchair the whole day!!!
My baby knew how to turn suddenly one day when she lied on a playmat (the soft round shape mat for small baby to play inside)... even the baby's bed is not big enough for your baby to turn his body... so it is better for u to place your baby in a playpan so u don't have to look after him when he is alone... each of your twin baby can put in a playpan for a while each day...