Children are individuals and it could be that one would prefer to be in a state school and the other would not. Assuming that is not the case, I think it's important that you treat your children fairly, so if you cannot scrape together the money (including the grandparents' contribution) to keep both in a private school, then it would be best to put them both in a state primary. Our girls went to state primaries and they are now both in Russell Group unis.
You could always explain to the grandparents that it wouldn't be fair to treat the children differently and that nowadays girls' careers are just as important and necessary as boys, so thank them kindly for their offer, but say you will move them both to a state school. Maybe they will realise the error of their ways and offer to fund both. Or maybe they cannot afford to fund both. Even if you could pick which one got the funding, it would still be unfair to the other, so in the end, you're left with moving them both to a state school anyway.