“The last word: one sentence each.”

A set-piece. One table, two pens, the same evening, one joint artefact. These are the pages people photograph.

Why this question

No pressure. Only the final line of the book.

And once you’ve both answered

Write it, read both aloud, and close the book together. The archive stays open for additions indefinitely.

Answering it honestly

One table, two pens, the same evening. This one doesn't work by correspondence.
Argue about the details out loud before anything gets written down. The arguing is part of the artefact.
Finish in the sitting. A half-built set-piece has a way of staying half-built.

And the house rule, whatever the mode: leave a blank rather than write a lie. A blank can be filled later; a lie sits in the archive forever, and you both have to live with the paperwork.