Same Field In 2 Tables But Using One Form Same Field

I have created a number of tables and I am now moving onto forms, but struggling to find out how I can get data captured in one form to populate into 2 tables(if that is even possible).

Table 1- Inventory

Ok, so I have a INVENTORY TABLE where all the stock items, amounts and calculations are done. In this table I have a field called "Shrinkage" so putting in an amount in under "shrinkage" for a particular stock item would deduct it from the total stock amount.

Table 2 - Stock Shrinkage

I created another table, STOCK SHRINKAGE TABLE with the following fields - ID; Date; ProductName; Quantity; Reason; User ID.

So those are my 2 tables, I have created a SHRINKAGE FORM, using the same fields in the STOCK SHRINKAGE TABLE.

What I would like is when a user enters an amount under the "Quantity" field in the SHRINKAGE FORM, I want that exact amount to be stored in the STOCK SHRINKAGE TABLE under "Quantity" and also in the INVENTORY TABLE under "Shrinkage" for that particular stock item.