How Do I use data entry from recipient 1 for formulas activated by recipient 2?

I have a field where recipient 1 is tasked to enter data.  Let’s call this data “Subtotal”.  I’d like to be able to utilize the data in this field when a modifier is chosen by recipient 2 via checkbox.  Let’s call it “10% off coupon”.  Ideally, a simple formula field could calculate 10% off the subtotal when recipient 2 checks the box.  However, the issue that I’m running into is that the data entered by recipient 1 is not available to be used in formulas when activated by recipient 2.  How can this be accomplished?