Add & manage records in sub table from main table layout

I have the following situation:

Order Table

- Order number

- Order info

Sub Table: Print Data

- Order number (to create a link to Order Table)

- Print Data

Now, a single order can have multiple print data. Right now, I have a layout using the Order Table. With a Portal I can list all the Print Data. Next to the Portal I can display the print data for entry. The idea is that the printers can see all previous prints and swap between the Print Data in some way using the Portal.

My idea was to add a button in the Portal that sets a field in the Print Data table to a certain value, and use the relationship between the two tables sorted by that field value. The problem here is, that if I use this script several times, there will be several records with the same value so that won't work. I would need to set all the values in all the records back to default but within the portal, that doesn't seem to work. The idea is sound, but I can't find an easy solution to make it practical.