https://www.collaboraoffice.com/ is LibreOffice in the cloud.

Onlyoffice can also be self hosted as a cloud office solution (can't remember if that one is 100% FOSS).

Joplin has a sharing feature that allows multiple user editing of an entire notebook.

The downside is that both individuals need to have Joplin installed and either have Joplin cloud accounts or a self-hosted installation of Joplin server, which in either case may be more effort than is worth it for you.

Etherpad is a relatively lightweight option: https://etherpad.org

You want Gobby

We are using Hedgedoc a lot, and we love it!

Abiword and onlyoffice are good options too

if you're looking for just a collaborative editor then there's etherpad.

I like CryptPad a lot.

Collabora Office is nice too but I prefer CryptPad for my purposes

Maybe Etherpad?