Selling an add-on of limited quantity across multiple items

Site URL: https://www.revitalizingretreat.com/store

Hello! I am trying to create an add-on of limited quantity to sell across multiple items, and I'm not sure how to go about it.

We are a retreat business, and we have a limited number of room upgrades (qty = 4). There are two ways clients can purchase a spot in the retreat:

• paying full price  
• paying an initial deposit (and paying the balance at a later date) 

These options are both listed as items in the store.

Is there a way to create a room upgrade add-on with a quantity of 4 that is listed as an option on both of these items in the store? I don't believe adding it as a variant would work because a sale on one of the above items would not update the quantity available on another item. 

Thoughts or ideas about how to make this work? Thanks!

This worked. Thank you!