I just pushed a new release candidate.
Changelog
* NEW: Added option to logout customer.
* Tweaked: Order of setting customer ID from cookie and setting the cart key.
* Dev: Added filter to override cookie check for authenticated users.
Comparison from the last: https://github.com/co-cart/co-cart/compare/v2.1.0-rc.3…v2.1.0-rc.4
New release: https://github.com/co-cart/co-cart/archive/v2.1.0-rc.4.zip
Now you can logout a customer after adding an item and add an item for another customer. You see a cookie is stored on the users device that stores their cart key.
If this cookie exists then the cart will load the cart based on the cart key stored in the cookie.
The new logout option removes the cookie for the customer.