How to get through today? What can I do to help Dd1 and keep Dd2 safe?

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I haven't been on here for ages but really could do with some advice today .
Dd1 is 9 and has a genetic condition and Autism. She's had major issues relating to the genetic condition for over a year now and this can affect her behaviour severely . It's incredibly hard to cope at the moment and I'm not doing well at all .
This morning dd got agitated about Dd2 being on the sofa she wanted to sit on and when she didn't move she flew at her and attached her then me when I got in to protect her. We then had a scary hour where she was just flailing about attacking me and trying to break things,biting herself and throwing herself to the floor etc. This is not unusual for her as such but just a very bad one today. She's now lying down and says her head hurts. She is calm ish but I know the next time something upsets her she will kick scream and bite. Poor Dd2 I feel I am failing them both today.
I am thinking what the trigger could be and I think maybe too busy /stressful day yesterday (rare day out with family, planned around dd1s needs and a happy day but exhausting for her as physically she did much more than usual and she would have found being out all day tiring)
What can I do to make today get better from here ? Often dh and I take a child each so Dd2 gets out but I suspect this will go on all day. I need ideas to calm her down as this is way worse than usual. In similar situations do you just stay in and keep the upset /stressed child separate ? I just don't seem able to help her when she's in that total melt down state and it feels like it could start again any minute if I handle this badly .
Hope that makes sense I am shattered .

Oh god sorry that is mammoth.
The gist of it is -
Please help with any ideas for action to help a child in serious distress and violent with it.

And, how to keep today as calm as possible now.

I can't help with dd1, but can you just find a friend for dd2 to go to all day? If the parent of a friend of one of my children called and explained your sitiutation, I would be round to pick them up and keep them for as long as needed.

At least then you and dh can tag team with dd1, keeping yourselves calmer and that might help her too?

You poor family. All of you must be on edge and thoroughly exhausted! I have very little to offer in the way of advice. But you have learnt a lot. 1) DD CANNOT handle long family days out (2) as this is a 'usual' thing, then plan in advance for future meltdowns by asking potential support network to have DD2. Its such a difficult situation, but if there are both you and your partner to support each other through the meltdown and of course to support DD - you will get through it. They are horrible to experience, but tell yourself it wont last forever, ours are usually no more than an hour or two. Which leave us all exhausted and shell shocked tbh. My ds is very apologetic after, not in the verbal sense, but he asks what he can do to help me with housework etc. .

Can you set up two comfy areas? Pillows and cushions and blankets? The give one to each child. Make sure dd2's can be a safe space that can be protected by door, maybe a secret cubby under the stairs? And dd1 can use hers as a calm place to calm down if needed?

Thanks everyone for the help and sympathy it's all a bit calmer now so just reading through.
I could probably have Dd2 picked up or one of us take her out. Just so hard both of them want me especially when it's stressful.

Chow now yes dd1 went to get Dd2 a drink afterwards I think she feels bad . So hard on them. You are right we have learned . I think others just don't realise the after effects of seemingly normal things as often dd1 will seem ok at the time but even when she has enjoyed herself it's just too much before and after to be worth it.
Jinramen that's a good idea. We have a lock for the living room door for extreme situations if we need to properly keep them separate but obviously avoid where possible. Knowing my kids they would fight over whose space was whose of course !! We actually do have a cubby under the stairs actually so that could work.