Multiple variants deplete one total stock?

Squarespace doesn't have any built-in features to pair the inventory of different variants or individual products, and it's not possible to do with CSS.

There's a Squarespace Extension called Trunk Inventory that can do this. It is reasonably easy to set up but you must pay a reasonably high monthly subscription to use it ($35+ per month), depending on the number of orders. 

Trunk works by using Squarespace’s Inventory API, so it possible for a developer to build their own solution on non-Squarespace infrastructure or use an RPA tool to manually build a solution. However, these would require a good understanding of APIs, API keys, and/or code. You’d also need to be able to maintain and support it on an ongoing basis - hence the costs.

There's an existing Circle thread here too (Circle members only):