i was thinking that, hocus. this is just an oversight because of the statemented child. if both were nt then they would have been allocated the same school as a matter of course - it's just an administrative error.
on a slightly more practical note though - i would suggest just getting your child's paediatrician to write a letter for the appeal panel confirming that your twin with sn will find it extremely distressing dealing with a new school setting without his brother, which will be detrimental to both his health and education, and is easily solved by allocating the multiple a 'PAN +1' place for this school year.
we've used a similar argument in reverse, as my statemented child is younger, but was familiar with the school because of her siblings attendance there - not a sib application though (infant school/ move to out of county juniors). so in order to get named school best for child etc. at the time she was not independently mobile, and so the obvious choice wasn't an old victorian building
. it is now an accessible old victorian building though. 
the only difficulty i see is if the statemented child is already in a 'PAN +1' place. if they have included him in the named school within the PAN, i think your appeal will be fine.
but get a paed's letter backing up the necessity of having both multiples in the same school, for the wellbeing of the sn sibling. so your appeal is based on your sn child really, even though it's the nt sibling that you want a place for. 
more than one way to skin a cat, etc etc.