Attribute stock is separate to product stock, their quantities are not shared directly like this. I suggest you read more about how it works in our documentation.
Only the full version will affect product stock availability on the frontend. The free lite version does not affect product availability or prevent over-purchasing, as mentioned in the description. It can only be used to track stock quantities internally.
If you already have the full version please reach out via our premium support channel and I’ll be more than happy to guide you on how to configure shared stock for the double seat.