By default, the plugin allows customer to put multiple pre- order products in their cart at the same time:
If you wish to deactivate this possibility, all you need to do is to check the “Prevent adding more than one pre-order product to the cart” option, which you can find in YITH Plugins > Pre-Order > Settings:
Customers will see a message as soon as they want to add a second pre-order product, as shown below:
