I suspect what would happen is that if you were not allocated the place you wanted, for the twin without SN, you could then Appeal (schools admissions have their own appeals) and it would be extenuating medical and social circumstances to admit them (if it is a mainstream school) Also from a transport point of view the LA might save money if they did not arrange transport for the SEN twin, because you were able to take both children to and from school. This might be a reason for them to put pressure on the school to take both - otherwise they might legally be obliged to provide transport for the SEN twin if school not easy to get to or in a different direction.
Another factor to consider is that I think most LAs have an admission policy of admitting the second or third multiple if there is no place technically at the school - this is just normal admissions policy. You would have to look up the LA admissions policy or the schools admission policy...I can remember seeing it somewhere.
Most schools despite filling their places, will end up in Sept with spare places, and then resort to the waiting list. If your NT twin is on that waiting list, having applied to the school you want for the SEN twin, but been refused a place, and your SEN twin's place is confirmed late (ie after April, lets say July thanks to delay in the EHCP process) but he is still entitled to a place in that school (they basically have to admit the EHCP child unless they can argue forcibly it is not a suitable placement - quite difficult for them to refuse). So then once one twin is in, despite all places being full, your Appeal to the AdmissionsTribunal having been successful, would mean that a waiting list place,when it came up, would be freed for your second NT twin.
Even the most oversubscribed schools end up with some waiting list places in Sept/Oct - I say this to reassure you that even if in the first instance your second twin is refused a place, one may become available.
Third point I would like to make is that, siblings often do go to different schools. I have twins, one with SN (HFA) and although they went to primary together, they did end up in different schools more appropriate to their needs (both mainstream though) It may seem like a disaster now, but twins are not usually put in the same class, and can often flourish in different settings. Your NT twin may like having their own space, as may your twin with SN. My twins suffered a bit from comparisons as the years in primary drew on. I don't know your circumstances and my twins were very attached, but despite the logistics you may find it works out to have them in different schools - lots of people have to juggle different schools as their kids get older. I now have three kids in THREE different schools.
I would advise booking an appointment with IPSEA over the phone or failing that, they are very busy, email them - they can come back with legal stuff associated with SEN admissions. Tamba also seem to do a SN facebook group where someone might have experience...I haven't used it ever, but just googled and noticed it.