The expenses for boarding schools would be more than $70,000 for each of your kids every year. For day programs it would be around $55,000 a year. The stipends for PhD students range between $27,000 to $50,000 a year, and the visa issued to the spouses of most international students (F-2 visas) do not allow them (the spouse) to work. So that stipend will likely be the sole income for your family.
Unless your husband has been accepted to a PhD program, there is really no point in looking at schools for the kids. Depending on the field and the programs that you are looking at, acceptance rates to PhD programs vary between 20% and single digits. There is also a correlation between how challenging it is to be accepted, and the annual stipend. Universities and field that provide the highest stipends are also the ones that have the lowest admission rates.
Like PhD programs, boarding schools which would provide enough financial aid that you could afford the school on a PhD stipend have very low acceptance rates (generally 20% or lower). So there is absolutely no guarantee that either of your kids will be accepted to a private boarding/day school, much less that both will be accepted to the same school.
It just seems to me that your present plans have too many moving parts, and too many things that, if they don’t work out, result in the entire plan not being viable.
My advice to you would be to first determine if and where your husband will do his PhD, what his stipend will be, where you could afford to live on that stipend, and what the public schools in that area are like.