Hello! I've checked with an agent from AXA whether she could help, here's what she told me :)
The doctor's memo has to be very clear that Twin 1 has fully recovered and will not result in future complications/illnesses. Some illnesses don't allow you to be covered by insurance even if you've fully recovered, because it can lead to other illnesses.
Eg. You may have survived and fully recovered from cancer, but you won't be able to get insurance coverage anymore because cancer could lead to further complications/illnesses. Or things like high cholesterol? It can lead to heart diseases and even if you fully recovered, there's already the medical history there!
As for Twin 2, she mentioned that some companies may not want to cover because genetically they might have the same issues in future - although Twin 2 is okay during birth, the fact that they're twins puts Twin 2 at higher risk of having the same genetically speaking.
If you bought prenatal insurance, you can try to see whether you could be covered under exclusion or temporary exclusion - if you didn't get prenatal insurance then it's probably gonna be quite hard for you to buy any for them now :/
You could probably try to get the doctor's memo stated clearly about it - whether she's fully recovered, is it genetic, does Twin 2 have risk of getting it, any complications they may face etc. Then from there, check with agents? Jiayou!