Two babies is not twice one baby. It is a different session with a different rhythm.
Twins
Longer, always. Two newborns rarely sleep at the same time, and the frames everyone wants are the ones with both settled together. That means waiting for an overlap, which can take a while and cannot be hurried.
We plan a longer slot for twins as standard, and we photograph each baby individually while the other is awake, so the session is productive during the gaps.
An older sibling at a newborn session
Do the sibling frames first. Always. A three-year-old asked to sit gently with a newborn has a limited and non-renewable supply of patience, and it is largely spent within the first fifteen minutes.
After that they can go and play, and the newborn work continues without an audience. Trying it the other way round is how sibling photographs end up looking coerced.
The second-child problem
Worth naming, because it is nearly universal. First children are photographed constantly. Second children are photographed while someone is also supervising the first. The result is that the second child grows up with markedly fewer pictures, and parents notice years later.
If that is you, the fix is simply booking a session — the same one you booked without hesitation the first time.
Photographing them together as they grow
Sibling frames get easier once the younger child sits, and harder again when both are mobile. The sitter stage for the younger one is usually the best single window for a picture of both, for the same reasons it is the best window for anything else.
Practical
Bring more spare clothes than you think, for everyone. Sessions with multiple children run longer and messier, and that is normal rather than a sign of things going wrong.