Are you thinking of letting your children take an exam for a grammar school or other selective school? Every parent wants their children to have the best possible education, but is this the most important criterion in your choice of schools?
What if only one child passes? Some parents contact Twins Trust to say that they thought it was worth a try just in case they passed, but they hadn't seriously considered the possibility that one may pass and the other(s) not.
If the most important consideration for your children is that they both/all go to the same school, then do think carefully before letting them take an exam. Talk to them (preferably separately) to find out what is most important to each of them.
If the primary school can provide written evidence that your children have had very similar test results for the past 2 or 3 years, then Twins Trust may be able to help you with an appeal, but there would certainly be no guarantee of success. So do think very carefully before making this decision.