AIBU to consider private school for one child but not both in these circumstances?

You are absolutely not unreasonable, you’ll be doing your best by both children.

I was going to do exactly this if my second didn’t get a grammar place. My eldest, who was in a super selective, understood why - basically the comps in our area are dreadful and we want the best for both of them. We didn’t need this in the end though but we had accepted the private school place in the January until the grammar offer came through in March.

Your daughter is bright, so she’ll get it.

I don't think YABU but do have some perspective of a family home where one child went to a private school (me) and the other (my sibling) did not.

Private school in itself worked our very well for me and I thrived there as a child in a lot of ways whereas I struggled in mainstream school for a few reasons (none of which related to learning disabilities) but there's no getting around that it absolutely altered the relationship and dynamic I had with my sibling. Private school even as a non-boarder is very busy with differing term times and in my case very different culture and more frequent extra curricular activities, as well as a half day on Saturday. I therefore very rarely saw my sibling and when we did realistically our lives where quite different - different hobbies, priorities, curriculum, different types of trips abroad, We often were holidaying separately. I have a different accent to my immediate family. We had entirely different friends who never crossed paths and so on. Of course not everyone will end up in these same circumstances or extremes of the same but I do think it's naive to go in to it imagining it won't affect dynamics at all.

As an adult my sibling has in fact done quite well for themselves but the opportunities afforded to us have always been quite different and usually on paper more beneficial to me as a result of links to private education and whilst we aren't close regardless I do wonder what their feelings are on that. IMO they also would have done very well at the school I went to.

As others have said it's not a magic bullet, I have lots of friends from school who felt very negatively about it, those who boarded especially. School culture differs widely but I find (having also worked in a couple at one point) that they can be very competitive environments, the success culture can be very high and children who don't achieve highly or meet the social currency (what your parents do, what your house looks like) might struggle with feeling as though they fit in.

Second getting DD2 an EHCP as soon as you can. She is coping now with good support from school but having that legal document in place detailing exactly what she needs will really help when she leaves primary.
It can take a long time if you have to appeal any stage of the process so starting earlier is definitely to be recommended.
Also is DD1 is "likely ND" bear in mind she may need extra support/accommodations through secondary. My eldest (ASD) was completely fine until year 8 then the wheels came off quite dramatically

I think it is fine to choose a private school for DD2 if DD1 gets into the grammar but it wouldn't be right for DD2.
However, a private school is not a magic bullet to resolve all concerns and I think you would be wise to take getting an EHCP for DD2 really seriously

I agree with @CoverLikelyZebra saying ^this.

DD2 needs an EHCP now. Don’t wait. Don’t listen to anyone saying it isn’t needed. It is. It doesn’t matter if the school is being flexible now. EHCPs don’t just cover ND conditions.

If DD2 can’t attend school full time, is alternative provision in place?

If your eldest is "likely ND" then will she be ok at the grammar? Or will she start to flounder once not at primary school?

If she’s on a part time timetable, what are the plans to increase that to full time. Part time schooling isn’t a long term solution surely. I would start the process of requesting an EHCNA because, if she struggles in mainstream as time goes on, you will need some options for the right type of provision.

I think it is fine to choose a private school for DD2 if DD1 gets into the grammar but it wouldn't be right for DD2.

However, a private school is not a magic bullet to resolve all concerns and I think you would be wise to take getting an EHCP for DD2 really seriously despite her currently coping OK if you already have evidence that she won't cope well in mainstream secondary. I know from experience that the process can take many years and if you wait until she is at secondary age and struggling, she may already be in y11 by the time you get anywhere. Pursue getting an EHCP more rigorously than you think is necessary - the system is designed to put people off so that only those with real determination/desperation get through. Managing school part time is NOT the same as being OK - she is entitled to a full-time education but isn't able to cope with full time exposure to a mainstream environment. That is VERY strong evidence that she will not be able to cope with a mainstream education at secondary level.

Has your youngest got an EHCP?

It would be the only fair thing to do

We have one in each for similar reasons. We explained it carefully to them - sadly the state options are too limited to suit every child & we are fortunate to have freedom of choice. It may come back to bite us in future with resentment one side or other but I was wiling to risk that over child 2 being f’d up by having to endure a damaging educational environment for his formative years.

Choose, as far as you can, the best school for each kid as an individual. Your idea sounds fine to me. Of course if the eldest doesn't get into the grammar, you have another decision on your hands....

How old are they both?