I’ve not got twins and am through it but hope sharing my experience may help
I’ve one v bright and one borderline
V bright convinced he fucked it up and I actually didn’t sleep for the six weeks we waited for the result, terrified he’d end up in a school totally unsuited to him
He sailed through
The other one failed one paper by one point - Kent. She was accepted for the school I think you’re talking about in the next town but she desperately didn’t want to go there, literally all her friends passed and we had 9 months of all her friends being delighted and her sucking it up whilst we waited patiently to appeal. She went to the other school for a streaming test a few weeks before the appeal and I had to collect her early from the head’s office as she was having a panic attack. She was very unhappy when I got her “none of the girls are like me, I hate it here” etc.
No pressure for the appeal then 
I have to say, apart from that day, I am enormously proud of how my DD handled it all. She was patient and showed huge resilience - she’s since won awards for this at both primary and secondary
She’s in Y8 at her chosen grammar now, appeal successful. She’s doing very well and is very happy
I was ambivalent about which school would be best for her but she was utterly unwavering in her choice and you know what? She was right
So if one DD scrapes a fail, I’d recommend appealing. It’s not for the faint hearted for sure, but our DD actually grew from the experience and I’m so glad we persevered.
I’d also consider the grammar in the next town etc - check which really is the best fit rather than which is the most local.