The thread, and why there is only ever one
One written exchange holds the whole arrangement, and what is not written in it is not part of the booking, however clearly the two of you agreed it.
The thread is the single written exchange in which a booking is arranged, held by one of you rather than by both. What is in it happened. What is not in it did not, and that applies to the two of you as much as to the house.
Why one and not two
An hour can only be agreed once. Two people writing separately about the same evening produce two accurate replies about two different arrangements, with no way to tell from the inside which one is real. It is the single most common way a booking made from two countries goes wrong.
Who should hold it
Whoever lands in Amsterdam first, whose data works abroad, and who is awake when replies arrive. Usually the same person answers all three. When they split, give it to the first to arrive, because the room number and the entrance are the pieces only that partner can supply and the pieces needed latest.
Keeping the other partner informed
Three facts, updated when they change: the hour, the length and the format. Forward the confirmation as text rather than as a screenshot, since an arrangement moves and a picture of it stops being true. The thread guide works through the whole method.
What only the thread carries
The amount of the deposit and the details for sending it appear in the thread once an hour is agreed, and on no page of this site, so the partner who never opens it never needs them. The thread is also where the clock on any change starts: a move or a cancellation is read from the minute the message landed in it. That is why a short line from whoever holds it, typed at the gate, beats a careful message the two of you agree on together an hour later.