Hi Emma,
Huge congratulations on your twin pregnancy.
I have twin boys who are nearly 7 months old, I always bathed them separately so didn't buy two bath supports I just had one.
But I found that one of my twins hated the support and preferred to be held whereas the other didn't mind either way.
I would just buy one to start with and see how you get on.
Im so jealous of you I really miss the newborn days they were so hard but so worth it they just look so cute snuggled up sleeping together.
x