Sync files between 2 macs I want to synchronise files normally stored on a desktop Mac - which is in use by my customer's partner under his AppleID, while using my laptop, logged under my customer's AppleID, while away from home.

What is the best way to do this?
You can use a 3rd party online storage and synchronization solution such as Dropbox.

Dropbox also offers support for shared folders, so you can have two separate Dropbox accounts one for your customers partner and one yourself, while having folder(s) shared among the two accounts.

Dropbox has native macOS, Windows and Linux clients available along with mobile apps for iOS and Android.

If you plan on creating new Dropbox accounts, consider leveraging Dropbox referral benefit which will offer additional bonus to both referrer and referee.