Add stock field to product display containing multiple SKUs
First, you will need to add an entity reference field to your line items. This will reference the product display node.

Then using Views Rules create a line item view with a "Rules Display" 


For "contextual filter" add Order ID.
For "fields" 


Add the quantity 
Add Node ID (nid) of the referenced Product Display Page.



Now in your rule simply add a view loop and pass the Order ID as a parameter. 

Inside the View loop:


Fetch entity from ID, pass the NID from the view result.
Do Calculate a value. Get the fetched entity quantity -  view result quantity
Set a data Value for fetched quantity field, calculated value.