If a field already has a value, how can I tell the next value to go to another field?

I have two forms:

The first shows a list of 100 products, each with a check box.  If a product is used that day, I check the box-- typically between 3 to 10 products used per day.  For instance, today I used Q,R,V, X, and Z, and so I check the corresponding boxes.

On the second there are 20 rows for inputting the keycode and quantity used for each product that was checked off on page 1.  For instance, when I checked off the box for product Q on the first page, on this page I specify that the keycode for Q is 1032, and I used 7 of them.  The next line says that R's keycode is 4402 and I used 2 of them, V is 5001 and I used 1 of them, etc.

I've got a simple script for autopopulating the corresponding keycode on page 2 when the checkbox on page 1 is checked.  HOWEVER, that only works for row 1.  So when I check the box for using product Q on page 1, on the first row of page 2 it automatically writes "1032" into the keycode column.  But when I say I also used product V, it just writes "5001" over product Q's keycode.

You can see my problem.  I need a way to tell the second (then third, then fourth...)  box I check that if there is already a keycode entered on row 1, it should enter the keycode onto row 2, and if that's also been filled, go to row 3, then row 4, etc., all the way to row 20.

It might not be elegant, but it would save an incredible amount of time.